Yeah you need to use the remote batarang, hold LT and press RB when it locks on a target. The grapple boost is not the zip line, but its where you grapple onto something and when you are getting close you push the grapple button again and you keep flying. It's not too hard, but just takes a few tries and you can generally get it flying around the city.
Edit: Actually I don't think you press the grapnel button I think you press A or something, I can't really remember.
Last edited by Joel78; 01-09-2012 at 12:57 AM.