One is that the cc is outdated or otherwise incompatible with your patch level. Another is that it requires a pack you don't have, although that's not as common with Sims 3, and it sounds like you checked. Yet another is if the .package files were not unzipped (still in .zip, ....
Solved: I was checking out my sims game when I accidentally pressed a couple of buttons and all my .package CC turned into Internet Explorer files..
floorplan - A Sims-Like Unity Level Design Plugin realtime-CSG-for-unity - Realtime-CSG, CSG level editor for Unity RoadArchitect - Road Architect for Unity PrefabPainter - A basic prefab painter for the Unity3D game engine roadbuilder - Easy-to-build realistic roads in Unity. Foundation of...