I cannotfigure out how to plug my bass into the computer and have sound come out. Do I HAVE to use an interface? I thought that with apple, an interface is not needed. They tried to sell me an M-audio device. Last night I brought up Garage Band and I plugged my bass into the side jack next to the jack with the icon, "head phones". I also changed my settings to line in under the Sounds page. Any one know?
Thanks

Not sure why it wouldn't be working for you. If you have a particularly low output (old passive pickups) you might need to boost it with a preamp like the Griffin iMic. Otherwise, the mic input's sensitivity should be sufficient to properly amplify your bass.

One thing to check for on your Sound>Input pane is your input level setting. Make sure it's up. Always double-check your interconnects too. Sometimes an ill-fitting quarter-to-eighth-inch adapter can mess it all up. Try different cables and go slowly and methodically until you get it. Good luck!

Also, In garageband, in the bottom right corner when you are setting up your inst, check the MONITOR box.

Three things

I understand you are using an adapter to plug your bass in. To get a clear signal, it is essential to use a standard bass cable and use a 1/4 to 1/8 adapter, not the other way around. Headphone jacks are waaay to sensitive to EMI.

Your bass has a mono output. Your adapter, although probably stereo, will lead a mono signal into your line-in plug. That means you should select line in - left (don't remember the exact way it's written) in your input choice. It will fill both channels with an equal signal, instead of trying to use it as a stereo input with nothing happening on the right.

If you want to hear something while playing, make sure you have activated monitor mode for your bass' real instrument track. Very important.

Three things

I understand you are using an adapter to plug your bass in. To get a clear signal, it is essential to use a standard bass cable and use a 1/4 to 1/8 adapter, not the other way around. Headphone jacks are waaay to sensitive to EMI.

Your bass has a mono output. Your adapter, although probably stereo, will lead a mono signal into your line-in plug. That means you should select line in - left (don't remember the exact way it's written) in your input choice. It will fill both channels with an equal signal, instead of trying to use it as a stereo input with nothing happening on the right.

If you want to hear something while playing, make sure you have activated monitor mode for your bass' real instrument track. Very important.

+1

plus check your system preferences to make sure your line In signal is Hot (Strong) enough and to make sure that the Line In source is the one chosen. You also need to select in GarageBand for that instrument that the instrument is coming in off of Line In.