Hello Jon,
Thanks for the interest and welcome to the forum.
The 'Composing pieces off-line from a MIDI sequencer configured conventionally (Hauptwerk as a ‘slave’ MIDI device)' sub-section of the main Hauptwerk user guide (pages 189-194 of the current v4.2.1 version) has step-by-step instructions for how to use Hauptwerk in conventional MIDI sequencer environments, with MIDI from an external MIDI sequencer playing the virtual Hauptwerk divisions using MIDI channels of your choice. Hence please try setting it up as described there (especially with the MIDI ports ticked in the *left-hand* 'Console MIDI IN' column on Hauptwerk's 'General settings | MIDI ports | MIDI IN ports' screen tab, and with nothing at all ticked on the 'MIDI OUT ports' screen tab, and nothing at all ticked in the 'Sequencer MIDI IN' column).
Does that solve it?
If not, what MIDI interface are you using for your Hauptwerk computer, and how is connected (e.g. USB)? Is any other MIDI software running on your Hauptwerk computer at the same time as Hauptwerk?
Could I also just check whether you're using the Free Edition as a means for evaluating Hauptwerk with a view to buying a licence in the relatively near future? (If not, then of course that's no problem at all, but we don't provide formal technical support for the Free Edition, so I'll move this topic to the Free Edition area of the forum in that case.)