. Download the "io_TS3_geom.zip" and put it where you can find it.
2. Start Blender and go to File->User Preferences(or press CTRL+ALT+U).
3. Click the "Add-Ons" button at the top of the User Preferences window.
4. Click the "Import/Export" button on the left, then click the "Install Add-On" button at the bottom.
5. Navigate to where you extracted the file and select the "io_TS3_geom.zip" and click "Install Add-On". Sims 3 GEOM Import is added to the the list.
6. Click the check box to the right of the TS3 Tools in the list. The importer is added to the File->Import menu, and the TS3 GEOM Tools panel is added to the Properties Editor under Scene