As I thought, your problem is not really a problem, hehe...
You have to use this:
event_alpha: Disables the alphanumeric keys for fast moving. If you have a keyboard encoder or a keyboard hack with some buttons remapped to alphanumeric keys, it’s useful to disable them.
event_alpha yes | no
In addition I configured any of my buttons to work as the "enter" key, so with any button I can enter the game:
event_assign enter enter or a or b or c or d
with an or for each of your keys
I found advmenu extremely versatile and elegantly simple.
The problem I had was that it was for a certain project and I didn't want to see the names of the games in the screen saver and didn't want to enter the screensaver game if I pressed "enter" since the screensaver pictures where not game pictures. There wasn't any option to setup that, so I had to touch the code a little...