IT SEEMS THAT THE PICTURES I HAD UPLOADED FOR VISUAL AID NO LONGER WORK.. IT NOW SHOWS A LINK.. IDK ABOUT THAT, SO SORRY..
[ORIGINAL 2013 SOLUTION]
2017 UPDATE: I have found new solutions! After purchasing a laptop and installing SAI, unfortunately, I ran across this problem again. If the tips below do not work for you please click this link and check out TWO new ones I have: NEW SOLUTIONS
2016 UPDATE: GUYS. YOU CANNOT GET PEN PRESSURE IF YOU HAVE A MOUSE. I'M SORRY. BUT YOU CAN MANUALLY ADJUST THE THICKNESS OF PARTS OF YOUR STROKES BY USING THE ERASER TOOL OR LINEART TOOL.
Also, this is not a guaranteed solution and if this doesn't work