How are your fingers!
MG = Machine gun.
The unit will charge up from the board, but won't flash unless you have the circuit connected to the shoot circuit. Its that CN9 socket at the bottom.
The cheap flash unit...is it a Heng Long one? If so it comes with a plug.
I am guessing that the flash unit you have has a permenant live (to charge the unit) and a seperate +/- to ignite the flash (eg discharge the unit, though fingers are a good substitute!). It probably shares a common negative with something else.
Can you post up a pic of the flash unit that you have, showing the wiring and the plug, if it has one?
I have an HL flash unit in my Jagdpanther, if yours is the same I will be able to tell you where it goes. I seem to recall that it had a couple of wires dangling off it, but I haven't looked at it since I installed it early this year.