Why cant I click the "sketch" button on my Variable Section Sweep?

I created a simple sweep and all I am trying to do is sketch the cross section. I clicked the sweep and its highlighted in red and labeled origin. When I click the sketch button, a prompt says "select any number of chains as trajectories for the sweep". Im not quite sure what that means. Is there something else im missing here? Im using Wildfire Pro/Engineer 5.0. Thanks to anyone who can help.