...Also, if you have a Macbook, iBook, iMac (or basically any of the non-pro computers) you'll need the Griffin iMic (adds an audio-in jack using an available USB slot). That can be found here:
http://www.griffintechnology.com/products/imic/ or you can find them for about 10 bucks cheaper on eBay.
edit: nevermind about the iMic. You have a Macbook Pro. You're good to as far as the audio-in is concerned.