when you download any sort of clothing for The Sims 2, it will require a mesh. The meshes are sometimes already in the game (the skinny you mention) or they require an additional download mesh (such as the bodybuilder version).
The needed parts all go in your downloads folder. The downloads folder is usually located here:
Windows:
My Documents\EA Games\The Sims 2\DownloadsMac:
Home\EA Games\The Sims 2\DownloadsWhen you install these items, they will appear in the clothing catalogue in Body Shop, Create-a-Family, and Create-a-Student (University EP); as well as when you take the sims shopping.
When you are creating a Sim, you can choose the custom clothing (it will be signified by a * on the picture preview), or if you want an already made sim to use the clothing, you go to a shop and buy the clothing item, then when returning the sim to his/her home, click on the wardrobe/armoire and change the outfit (uniforms are most usually to be found under casual/everyday)
What you need to make a "bodybuilder" sim is a matter of choice. You can find all you need in this section:
http://www.insimenator.net/forumdisplay.php?f=158