On the menu bar their is a toggle (little icon/box) that shows all the brush pallet options, open it (double click) then  click (check the box) on "OTHER DYNAMICS" and you can set presure sensetivity. That SHOULD do it.

 

Oh ya... you have to be using a brush at the time or the pallet option icon wont show on the menu bar!

"Dance like nobody's watching; love like you've never been hurt. Sing like nobody's listening; live like it's heaven on earth."
  --  Mark Twain --