I'm attempting to install this program to the Sims Legacy and the first issue it ran into is that it cannot find the files. I can direct the program to the files, but do not know what to put in for the Registry. Does anyone know what pathway to use?

The registry entry is pulled from the INSTALLPATH (I think that's the spelling) entry in the registry. You can't actually populate that field in the program (at least not without editing the registry). That field just allows you to reset the program's path to where the registry says your Sims game is. Sims Legacy does not create that entry in the registry when installing.

Are you asking for help editing the registry? Does the program not work if you simply point it to the files?

VerDeTerre, are you talking about the "Location of the Sims found in the registry" line? As far as I understand, it's important to have the right game path in the "Location of the Sims used by SimCategorizer" line. So just put your game path there.
I don't have the Legacy edition, but the info from the other people here is the following:
If you own a copy of The Sims Legacy on Steam: C:\Program Files (x86)\Steam\steamapps\common\The Sims Legacy Collection
If you own a copy of The Sims Legacy on the EA app: C:\Program Files\EA Games\The Sims Legacy

Yes, I cannot run the program without filling in the information asked about the registry. I'm not up for it right now, but I think I could try to put in the game file location in that field and see what happens.

There were actually two areas asking for location - the one you are giving was the second one which is asking the location of the files. The first one asks for the registry. I do not remember this from before and I think that's because it could find the pathway easily. With the Legacy edition, everything has changed.

Well, I don't think that the "Location of the Sims found in the registry" line asks you about anything. It tells you the information that the program was able to find without your help, reading it in the registry, and I don't see the option to edit it somehow. I've just tried to delete my game path from the registry and it still doesn't allow me to edit this line, but suggests to direct the program to the game manually instead. The second line doesn't really ask anything either, but gives you the opportunity to make the program use any other location then the one from the InstallPath key. There is the note there telling you that if it's impossible to find the location in the registry, you can use the second line to direct the program to the game manually. This option is specially created for some game versions without the InstallPath key, so I don't understand why it doesn't work without it. What really happens if you put your game path there? Do you run the program as an admin?

Sorry to take so long getting back here. It was a pain, so I avoided it.

My game crashed and I reinstalled. I just put the categorizer program in again. Nothing loads and there is some sort of a message about not finding anything, but under "options" I can "set the path". There are two boxes to fill in, both pre-filled with "Error". The first one asks about the registry. I cannot type over the "Error" message, nor can I delete it. The second box asks for the location of the files. I can easily find the downloads folder from there. Running the program as an administrator does not impact it.

OK, these are the two boxes that we are discussing. I still don't think that the first box is there to fill in, at least not for the player to do it. The program does it itself if it's possible. If not (i.e. if there is no game path in the registry), the player still can't do it and really doesn't need to do it. This box just shows you something, not asks about anything.
The second box works differently, here you can find the location of the files. So you do it and what? The box still shows the error message and doesn't use the path that you find for it?

Right. I fill it in and nothing happens -- I get an error message:

"GetRegistryString failed on error. The path to The Sims can not be found. Please use the Set Path option on the Options menu to locate The Sims."

In the past (with older os systems, older security, and a different set up from Legacy), I think both fields were filled in automatically and the program was able to load files.

In newer OSes it will also be filled in... as long as you aren't playing The Sims Legacy Edition.

On a 64-bit system with a US English copy of the original retail edition of The Sims installed, the location Computer\HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Maxis\The Sims will have an InstallPath registry entry pointing to where The Sims is installed on the computer. (For instance, mine points to C:\Program Files (x86)\Maxis\The Sims because that's where my install is at.)

...I am half-tempted to create a Windows 10 virtual machine and add Edith/a dev build (because it also doesn't come with a registry entry--more sensible in my case than buying a game I already own) and SimCat to it just to see if I can replicate the error... 🤔

...I suppose I could also just temporarily delete the InstallPath from the registry and start SimCat to see what happens... *backs things up in case something explodes*

EDIT: I cannot replicate this error by only deleting my registry entry. If I open SimCategorizer 3.6.4 and use "Find" to find either copy of The Sims on my system without an InstallPath present in the registry, then click okay without touching the (ERROR) field for the registry in that window, close SimCategorizer and reopen it, it successfully displays objects from that The Sims folder for categorizing. This may be because I had opened it with a valid path or for some other reason. Might play around slightly longer to try to trigger it.

EDIT 2 (THE RELEVANT ONE): I get your error on SimCategorizer startup if my registry does not contain any data in the SimCategorizer registry entry for install location AND if I delete InstallPath from my Sims registry folder. If I use the Set Path option in the options screen to set the path, though, the setting immediately gets the relevant object folders from the installation folder I picked and I experience NO further errors, even on close and restart of the program.

I tried to delete the InstallPath info too several days ago, but had no problems with Sim Categorizer. The Set Path option worked as it should. But I have the old XP system and the regular version of the game.
Well, I would suggest to add the InstallPath key to the registry and to see what happens.