So, to clear up some of these posts that i'm seeing---
The button in the middle of the arrow pad, the return key, and the button under the scroll wheel are all the same thing. Also, the "up/down" of the scroll wheel is "up/down" with the arrows.
Now, if you have a Nes emulator, do like i do, set the "return" button to "a." This, in turn, sets the center arrow button to "a" as well. Therefore, i can play Super Mario Brothers with one hand

(not as fast cause i'm not using a "b" button, and i cant throw fireballs, but for standing in line for a quick play, its quite righteous ;)
These are 100% continuous reactions to the buttons being the same... for instance while playing Mario, the scroll wheel does not let me jump or use "up/down" at all.... i suppose its all dependent on the coding per program.
However, for most things, the buttons are congruent with one another.