Hello Olly,
Thanks very much for the interest in Hauptwerk.
On the right-hand side of the 'General settings | Audio outputs' screen in Hauptwerk, please check that:
1. The channel format is set to stereo (not mono), and:
2. You have your Mac's built-in headphones output selected as the audio output device, and:
3. You have both its left and right device channels selected (as opposed to having 'none' selected for either of the two channel settings).
Does that solve it? If not, is the right-hand speaker in your headphones working for other non-Hauptwerk use (such as playing music in iTunes)?