Ok, so as far as I can tell from the docs that keyboard
- sends MIDI out its USB connection to your computer [which it sounds like you already have working],
- and sends audio [its own internal sounds] out the L/R 1/4" [audio] Outputs on the back
- To play/record one of GarageBand's own instruments, make a Software Instrument track and select the Instrument you want in the Library [on the left, shortcut Y] -- MIDI data from the keyboard will trigger the notes from the internal Software Instrument and be recorded in that track, and the sounds during playback will still come from the internal instruments themselves
- To play/record one of the keyboard's instruments, plug the keyboard's [audio] L&R Outputs into two line-level [1/4"] inputs on your audio interface [say, the smaller jacks in the middle of the larger XLR connections, like for Inputs 3&4 on your interface]. Make an Audio track in GB, and select the interface inputs you've plugged the keyboard into [for example, inputs 3&4] as the input[s] for that track [in the Smart Control panel at the bottom, shortcut B] -- an audio waveform will be recorded in that track, just as when you record a live [acoustic] instrument with a microphone
As LoFo also pointed out, the key to understanding this is to understand the difference between audio signals and MIDI signals -- hopefully the previously-recommended course will help explain that..