On newer versions of Gimp the Brush Dynamics window will not look like that. Here is how it looks in Ubuntu 13.04:
If you click on the notepad to the right of "Pencil Generic copy", it will show the menu where you can select what pressure will do to the picture (opacity, size, etc.). But it is all disabled/greyed out. What you need to do is click on the crazy blue arrow, then click on the crazy blue arrow again, which will open the brush dynamics selection dialog. There you can select a brush and duplicate it. You will then be able to select the duplicate brush you just create, click on the notepad button, and then finally edit the brush dynamics as shown by htorque.
This has been tested in a Thinkpad X61 tablet running 13.04/raring and Gimp 2.8.4.