You actually can't...
Well, you can make a button that toggle its aspect (use the "A" suffix instead of the "D" one in the button file name and once you click the button the A image is shown and when you re-click on it the N image is shown again), but you can't assign a button multiple actions (Play and Pause are different actions and cannot be assigned to the same button..). You can use only the Pause action to Pause/Play a file, but you can't use it to Play/Pause/Play since the file must be started once before pausing it and then making it to play again..
You can select the option "Auto play" in general settings, on use the PLAY image for the N file, and the PAUSE image for the A file and use that button to toggle the playback, but if you press the Stop button, you'll not be able to start the playbeck from the Pause/Play button, you should use the keyboard shortcut "X" or add another button for the Play only function.
I don't know if I was clear, but if you need more help or much explications ask here
