Seems that we're on different systems or ways of doing things....on my spektrum DX6i I control the turret and elevation with the throttle/rudder stick. Now what I did was first set Tamiya dmd/mfu as per instructions....then I went into the sub trim menu and set throttle to +55, after that was set I could fire my main gun but not my machine gun so I then put my throttle trim all the way down (and it is set down all the time)......so now when I push the throttle stick forward my main gun fires and when I push my throttle stick down the machine gun fires.

Now on other systems like a 4 channel transmitter (maybe that's what you have) then you do have to "Full trim in one direction, then flick the stick the same way to get the Main Gun or MG to fire" (As per Tamiya instructions). I don't have to do that with my system. If your coming to DAK's Battle Day March 7th I'll show you.