[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 for you I am so sorry. I am working on an updated solution so please don't fret.
Hey! Two years on I'm gonna make an edit. Paint tool SAI does not require purchase. The trial is good, too. However, if you've found a way to download it for fre