The Sims 3
House package files should go in your The Sims 3\Library folder; any other packages for custom content in the download zip go in The Sims 3\Mods\Packages:
If you have difficulty getting items you've downloaded to show up please see these external links for futher information:
To add a custom dormitory to your University neighborhood find an empty lot of the right size, set it to a community 'No Visitors Allowed' lot, then place the dorm from your Lots & Houses bin.
This lot does not contain any custom content.