I love those dresses for little girls but the mesh is not included in the file.
Does anyone know if it is a new mesh or if I could find the needed mesh elsewhere?

The "Miss You" dresses by Tea Leaf for quick reference.

I don't know if you're still looking for this, but I believe those dresses use the Maxis mesh for cfbodydressshortaline.

If you don't have a default replacement for cfbodydressshortaline, or if you use one that replaces the textures only, you can simply put those files by Tea Leaf in your Downloads folder and the dresses should show up. If you have a default replacement that changes the mesh, the dresses won't show up. In that case you can download the mesh separately here (Azaya's post).