Rich,
The easiest way is when you set up the DMD per the Tamiya instructions, when you get to the elevation stick only move the stick about 75 to 80% of it's travel in each direction. Once you have done that it is simply a matter of moving that stick quickly up or down and you will be able to fire the main gun or the machine gun depending on which way you move the stick.
Originally Tamiya would have you move the trim controls to fire the main gun and the machine gun, but when the newer radios went to digital trims, it was near impossible to move them back and forth quickly enough for battling.
Since you got the machine gun to fire, you can try moving the trim control in the direction to fire the main gun and you may find it will work. But the most reliable method is the first one I mentioned. As far as moving the firing to the flap switch, you need to be able to mix the elevation channel with the flap channel. I don't have a DX5 so I am not familiar with that radios capability, but I am sure someone else here on the RCU has you radio and uses it for tanking and may be able to help.
I believe if you search the RCU or the web you will find the mod you seek. I think I read about it some time ago albeit for another radio brand, but the procedure is basically the same. As I recall they went inside the radio and wired the push button switch to the elevation potentiometer (pot), and connected it so that it electrically maxed out that pot in the correct direction to fire the main gun.
Good Luck in your quest.
Steve