Hey clucas,

  Make sure you have the latest Line 6 Drivers installed:

  MAC Drivers : 7.1.0

  http://line6.com/software/readeula.html?rid=4171

  Since you have already assigned the GuitarPort as the audio device for GarageBand, the next step is to create an audio. You then will need to assign the input of this audio track to GuitarPort. Make sure the track is record/monitor enabled to be able to hear and record what you are playing.

  If you are unfamiliar with how to do this within GarageBand, review the included user manual within the program.