Hello Keith,
Could you first confirm that you've done each of the following?:
1. Connected a MIDI lead from your Orla's MIDI OUT port to a MIDI IN port on your computer?
2. Made sure that particular MIDI IN port is ticked on Hauptwerk's 'General settings | MIDI ports | MIDI IN ports' screen?
3. Loaded St. Anne's, right-clicked on the virtual Great keyboard, selected 'Auto-detect MIDI ...' then followed the on-screen instructions to press the highest and lowest keys on your Orla's Great manual? Does the 'Done' button light up (which would indicate that Hauptwerk has heard suitable MIDI messages from the Orla)? If so, if you click the 'Done' button, does the virtual Great play properly from your Orla's Great manual?