I found the specs of your CF-2a and see that it is MIDI ready as most later built organs are. You'll need a USB MIDI interface which has 2 male MIDI plugs (MIDI IN & MIDI OUT) on one end and a USB on the other. You can shop Amazon or your local music store to purchase one. I've had good luck with this one as it does't require any drivers, it was plug and play (based on Windows experience).
https://www.amazon.com/iConnectivity-mio-1-out-MIDI-Interface/dp/B00CO5IRSG/ref=sr_1_fkmr0_1?ie=UTF8&qid=1531933543&sr=8-1-fkmr0&keywords=mio+USB+interface
Since you are starting out, just map (auto detect in HW) your keyboards, pedal board, and swell pedal to the virtual organ you'll be using. Select stops via the virtual controls (touch screen or mouse). After you get the gist of how this works, you can move on to map the pistons and stop rocker tabs on the Allen to the VPO. Of course the native Allen voices will also sound. You'll have to figure a creative way to silence them. The easiest would be to disconnect the amps rendering the speakers dead.
For audio, most Macs have superior D/A (digital to audio processing) compared to a comparable Windows computer so you can start with the Mac's onboard headphone or audio output jack connected to your stereo or good quality headphones.
KenC wrote: can Hauptwerk play back through the Allen speakers?
Yes. Provided there are audio input ports available on the Allen to accept a connection from your Mac's output jack . You'll need stereo cable with the correct plugs (maybe an 1/8" stereo to RCA (left and right) plugs?). If not, there may be a way to rig a connection to the Allen amps. Regardless you may be disappointed with the sound of the internal speakers as they are designed to work with the Allen voices where as the HW voices are *far* superior in fidelity, quality, and sample length.
The usual progression (not in a strict order) for getting the best fidelity for your HW project is:
external stereo speakers
external stereo speakers w/sub woofer
high end (audiophile) stereo speakers and or headphones
better sub woofer
- - - professional, external, dedicated audio interface unit (USB/Firewire/Lightning) - - - -
studio monitors
multichannel audio with many studio monitors
Hope this can get you started,
Danny B.