You should read the WME Wiki. Otherwise you will get lost in keycodes, eventhandling and stuff. I already wrote what you have to do:
In the Button you make something like
on "LeftClick" {
Game.ApplyEvent("Keypress");".
}
Then you have to edit the Keypress section in your game.script file. You then have to check in which scene you currently are, and then check the keys. That way you can check both keypresses and mousclicks on your buttons.
Greets,
Spellbreaker