What software are you using. Regardless, you need to change the device in your Macs system settings as well as in the softwares settings. What interface are you using?

Sounds like he's using a mixer, running a line from that to the mac, and he's using Garageband.

Check the garageband settings.

If all else fails, get an XP machine and a hammer.
Note: do not apply hammer to XP machine. Apply with gusto to offending mac.

I'm in a school. This is not my set up. And yes, everything Codemonkey said was right about the set up.

I was looking around in Garageband and I could not find where in the settings it determines what line-in it uses. I'm very unfamiliar with it. And Macs in general. But I did find it in the system preferences on the Mac, with no results despite what should have been an easy fix.

GarageBand menu -> Preferences -> Audio/MIDI -> Audio Input

If there is nothing other than "built-in," reinstall your drivers for your interface.

it sounds like you're taking the line out of the mixer and sending it to the line input of the mac. if that's the case, then you don't have to change the preferences in Garageband; you actually have to go to the system preferences in the mac. Once there, click on the "Sound" icon and switch the input from the built-in mic to the line in port. that should do the trick.

-d.

AnomalyAlecB said (twice):
"We went into system preferences and clicked on line-in for the input, but Garageband is still only using the built in mic."

codemonkey: good call, i misread the post. i apologize.

if that's the case, then is there a problem with the line out of the mixer? it should work if you've selected it in the system preferences and garageband is set to built in audio.

-d.

Peterhunt was right.

But I just figured it out on my own about 4 minutes ago. But thanks for all of the help.