My answer may be a bit dumb but you need to go to pattern edit mode, then shift+stop/cont to select the trigger as you do for other instrument then write your trigger on the 16 steps.
So i'm guessing you did it and nothing comes out from the trigger output?

I've modded my trig out to get 12V instead of the original 5V so i've investigated a bit around it, you can check this thread to see which part of the circuit is involved (viewtopic.php?f=26&t=1125). Anyway, to resume, the atmega (pin 38) outputs a 5V S-trig (it goes from 5V to 0v at each trig), it's then inverted by Q66 and the related resistors to get a V-trig (that stays at 0V and goes to 5V at each trig). So I would first check if the atmega outputs the trig properly, and if that's the case, there something wrong around Q66, either a wrong resistor value or a bad 2N3906.

aaah that was it!
I did find nothing in the manual and oversaw the text below the button.

thanks a lot!