Joanie, the zip line is an automatic thing. All you have to do is get close enough to it and then hit your action key (the default being the "E" key). The game takes over for you and has Indy ride the line. You don't physically need to jump, whip, or perform contortions. If you hit "E" and Indy doesn't do anything, move him around a little to get to an optimum point.