You trigger it via MIDI. There are several ways to do this. The easiest being have your Mac near the GrandMA, plug in a MIDI interface into your mac and run the MIDI OUT cable into the back of the GrandMA into it's MIDI IN Port.
Make sure MSC is enabled on the board in the settings.
The MSC cue in QLab is configured with:
Device ID: (have it match the setup in the MSC preferences in the board, usually 0 or 1)
Q Number: The number of the cue in the GrandMA
Q List: The Executor Number for the GrandMA list you are working with
you can ignore Q Path.
That is how I do it, except I use a different MIDI IN set up.
Hope this helps.