Some sort of interface to interact with your MacBook.
Line6 makes a TonePort that's pretty cool if you want a ton of tones and don't mind going direct. If you like what tones you have at this moment, you could look at a Firewire or USB audio interface (you can either go direct into them and use Garageband for tones or use a mic and mic up your amp the classic way). I used a Tascam US-122 for a while until I finally chalked up the cash for an Apogee Ensemble.
Garageband is ridiculously easy to use. Once you download drivers for whatever interface you're using, you just go into preferences and under the Audio/Midi tab, you can choose your "Audio In" where you'll find your interface device.