I had a helluva a time with this at first as well. You may be doing what I was...aiming at a leg and hitting the secondary weapon button (nothing happens).
Well, its a TOW cable...i.e. it comes out the BACK of your buggy. So you have to fly past it, generally close to one side. When you feel you're roughly lined up with the back leg, hit the button. The camera angle will change to something that will probably cause you to crash the first couple of times, but you get used to it.
Now, this doesn't work 100%. I've played these levels several times and still don't always get it on the first try. You're probably on Correllia right now, you'll get a LOT more practice when you get to Fest! I can't for the life of me figure out how anyone finishes that in the required time for gold (I keep going 14 seconds over).