do you have the wobble made already? and all you need to do is add a speed filter to it?
try highlighting the track, and go to the "edit" section on the far right. click "add effect" then go to "automatic filter" and then click "manual".
then, go back over to you're track and click "add automation" and choose "speed"
try not to take the speed too high, as it will distort the sound.
hope this helps. if you are more of a visual learner, try checkin out the garageband dubstep tutorials on youtube. sometimes they dont explain exactly what button they are clicking or what they are doing, but you can just pause and go back to whatever part you missed.