You would use the PODxt as your USB Audio interface, in other words, you will use it as your main soundcard.
Plug your PODxt directly to your computer via USB.
Run Line 6 Monkey 1.23 and install all the updates in the updates tab.
If you don't have Line 6 Monkey 1.23 you can download it here:
Line 6
After you install the drivers and the do all the updates, you will be able to use the POD as your soundcard as your computer will have recognized this hardware.
Go into your system SOUND preferences and select the PODxt as your ASIO input and output device.
You will need to plug your computer speakers into the headphones out of the PODxt as you are using it to be your main soundcard.
Launch Garageband and go into it's preference do the same.
