First, create a free Apple developer account. Go to the Apple Developer Center, and click Register. Fill out the first page with your account information. The second page is a developer survey, which you can skip. Just go to the bottom and click "Continue".
Confirm your account and log in.
OS X 10.5
Download Xcode 3.1.4 from the Apple Developer Center.
If that link doesn't work, go to the downloads page and search for "Xcode 3.1.4".
If you have an older version of OS X, ask a TA for help. If you have a newer version of OS X but still want to use this installation method (not necessary if you are using the virtual machine), get XCode from the App Store.
You are now ready to for the Installfest!