helenn09 The Mods folder is only for actual mods and custom content. User-made worlds and lots are referred to as custom worlds and lots, and they are not mods or custom content in the traditional sense, so they're not treated as such.
Worlds with the .world file extension need to go into InstalledWorlds, and lots with the .package extension into Library, both in the same Sims 3 folder as the Mods folder. Files that are still in .Sims3Pack form need to be placed in Downloads and installed through the launcher.