First you need a male to male stereo cable that is 3.5mm (1/8"). This goes into the microphone port of your computer.
http://www.abccables.com/2ft-3-5mm.html
Then you need a 2.5mm to 3.5mm stereo plug adapter, because the Stackmat port is 2.5mm. The adapter will connect the Stackmat to the stereo cable.
Or you could get a stereo cable that is 3.5mm on one end and 2.5mm on the other end.
http://www.abccables.com/3-5-2-5-stereo-2014171.html
By the way for cables, male means 3.5mm (1/8") and female means 2.5mm.