You need only a few things to run Get It Together:
Simply unzip the .zip file to the directory where you want the program to reside. Then, either double-slick the .bat (Windows) or .sh (Linux, Mac OSX) file to run Get It Together. You may want to use the pre-made Windows shortcut for easy access to GIT from the Start Menu or taskbar.
If GIT doesn't even start for you, the first thing I would suggest is upgrading to Java 1.5 / 5.0 (two different names for the same thing). Even slightly older versions of Java have had some issues with starting up GIT properly. If the problem persists, feel free to send me an e-mail and I'll help you figure out what's wrong.
Good! I mean, not good for you, but it's good that the problem is consistent and reproducible. Either send me an e-mail or use the bug tracker to report the bug that you've found. This is the way open-source programs get better, is through the people who use them and find ways in which they can improve. Thank you for your help!
The Java-based MP3 player included with Get It Together (called "Java" in the Settings panel) only supports MP3 and WAV files. If you want to play your AAC files, you can either use the "external" player to play them (through Winamp or your favorite AAC player), or you can use the Quicktime player option (if it doesn't show up, then you need to make sure Quicktime for Java is installed on your system). Apple hasn't had the greatest support for Quicktime for Java, so I wouldn't count on it always working.
Yeah, I know. If I didn't answer your question, e-mail me and I will be glad to help you.