Apologies for the delay in response.
DTXM12 can definitely send MIDI information to GarageBand so you can trigger GB's internal Virtual Instruments, or record the MIDI data to play back through the DTXM12's sound module. If you're still having trouble with setting that all up, please reach out to your country's Product Support team so they can guide you and trouble-shoot in real-time. You can go to www.yamaha.com
, select your country/area and go to the "Support" page to view contact options. If you're in the USA, that'd be here
Please let me know if there's anything I can do from here.