You might need an amplified source for your mic before plugging it into your mac. I plug my bass directly into my computer using a 1/4" cable (with 1/8" headphone adaptor for the mic line in) and since it's an active bass I get a good level.
Try the mic and in Garageband start a new track. Make sure you set the track's Monitor to On and you record in Mono, Channel 1.