After you've downloaded 3viewer, open up the folder. Mac users double click on the Sims3PackViewer.jar file, Windows users the 3Viewer.exe file. (Windows need to have
Java installed.)
The program should open up. The first time it opens, I believe it asks you to choose the directory that you installed TS3 under. Once that's all taken care of, go to File>Open. Select the .sims3pack you want to install. Click the install button. Sometimes it asks me where I want it installed (can't remember if it does this on the macbook or not, haven't used it for awhile). If it does, just go to the Mods>Packages folder.
A .package file will show up there, usually with a long string of numbers as the file name. You can change this if you want.