Typical one button stick (Atari style):
1 Up
2 Down
3 Left
4 Right
5 NC
6 Button1
7 NC
8 Ground
9 NC
Typical two button stick (Sega Master System*):
1 Up
2 Down
3 Left
4 Right
5 NC
6 Button1
7 NC
8 Ground
9 Button2
* On the Genesis/MegaDrive Button1=B, and Button2=C. On the Genesis, pin 7 is used to select between layouts. Genesis 6 button pads have an additional chip inside to enable the extra buttons.
The easiest thing to do is directly wire the two button style stick. This should sufice for a majority of the games. Building the CD32 circiut is a bit more involved and could be left for later.
Oh, and the real thing to worry about is if the X-Arcade itself is doing anything funny. (ie. if it already has a multiplexer...) Do you have a pinout for the X-Arcade connector?