Documenting this here so that we don't have to re-learn it.
The trackball console acts as a mouse via the Ultimarc OptiPac USB converter. This converter drives the opto sensor on the Happ trackball and converts trackball movements into a USB mouse.
When connected to the MAME laptop, the trackball mouse works on the desktop but not in MAME by default. To change that,
- Run MAME without a game argument. This puts you into MAME screen where you can select games to execute.
- Hit <tab> to selet the Configure Options menu.
- Use arrow keys in the popup "Settings" menu to select "Device Mapping". Hit <enter>.
- Under "Trackball Device Assignment", set it to "mouse".
That's it.