[ORIGINAL 2013 SOLUTION]
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 free, I cannot assure you it'll work, hence me emphasising on the SAI MUST BE BOUGHT stuff. lol XD
Sometimes you dig too deep. SAI is sometimes a very complicating and annoying son of an apple, but maybe you're looking too far into it. As suggested some users said, "Unplugging my tablet, restarting my computer and re-plug