You can but it is unlikely to sound good as the speaker isn't designed for the range of sounds that you will put through it. If you can't afford some powered speakers, find a cheap old amp and speakers from a HiFi, I got a great eighties JVC setup with outstanding speakers. Speakers often sound better as they get older. If you wish to go ahead, here's how...
I'm assuming it's an instrument amp. You will need either a cable with a 3.5mm mini jack (used on most earphones) on one end and a 1/4" plug (used for instrument cables) on the other. You can also get a cable with the same plug on both ends and get an adapter to put on one end, as this will be easier to find. Make sure you check the balance to make sure you're not getting just the left or right side of stereo through the speaker as you might end up missing half the instruments in a song! Also, have your macbook on about two thirds volume to prevent distortion.
