One event command...Button Input Processing. Or something like that (Don't remember the name since I don't have XP on this computer). What that command does is according to the button the player presses it'll record it onto a variable. Say if the A button (Being the space bar and etc. I think) was worth 15 (Which is a close approximation to what the value is), it'll be recorded onto that variable you chose. Each button has a certain number assigned to it in Button Input Processing, and it's up to you to find that button number.
Now the variable issue itself, it's really easy. It's kind of similar to algebra, but not so really. All you need to do is find the variable value of that button, make a conditional branch on that variable being equal to the variable button value pressed and voila, you can get the end result.
Here's an easier more visual style:
Button Input Processing on Variable 0001
Conditional Branch: If Variable 0001 = 15 (15 is going to be that approximation number for accept)
Do whatever
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The button input processing notices the player pressing the button that uses 15, and if so then it'll modify the variable to that number...Hope it's understandable...