Mod the Sims has a very excellent step-by-step troubleshooter faq for exactly that problem. I would not want to steal their thunder, but the gist of it seems to be:
A) Make sure you are putting the packages in the correct folder. They do NOT go in Documents for Sims 3.
B) Make sure your custom content is not conflicting with your expansion/version. Compatiblity is the responsiblity of the creator of your cc to note.
C) You need a particular program to tell your game to look for the custom content. Again, Mod the Sims has it availible. They even have a little program that makes the right folder, adds the little program, and installs your custom content with a simple double-click.

You might try asking one of the experts in this matter, from where ever you get your custom content from.