First:-Make a new "instrument" by pushing the "alt","apple" and "N" buttons at the same time.
Second:-Choose "Real Instrument" (you can choose between real and software instruments)
Third:-Adjust the input on the bottom right (channel 1,2-they're both mono and Channel 1&2 is stereo. you need this if you have a bass with stereo output)
Fourth:-turn on the monitor as stated above,and adjust the input volume
If isn't working I don't know. I use the Garage band on my MacBook this way and it works fine!
IMPORTANT! Use a proper headphone to improve the quality,and plug in your headphone before all this. The sound would be delayed if plugging in after opening Garage Band...