Have you looked into the applications folder?
It has to be there.
If not: did you do a custom install? With a custom install you can leave out parts of the software package. I, for example, put away with iPhoto, as I don't need it.
You can always add software from your install disc.
I did that, but forgot the exact steps. Check your help files. You DO NOT want to reinstall OS X, you want to install just Garage band from the install disc. That's your theme you have to follow dealing with the help feature.