The Zip line is an extra charge when you buy the excursion for Bobsled Jamaica and Dunns River Falls. I did not do the Zip line but it's pretty much the same as most - into a harness and zip across the top of the trees and land on a platform. The staff at Mystic Mountain is very courteous and well trained in customer service and safety. I would suggest combining the Falls with the Bobsled. Give yourself at a minimum of a couple of hours at the Falls.
We had so much fun at Mystic Mountain - Bobsled, Restaurant and Pool - that we "short changed' ourselves at the Falls.
Hope this helps!