On our last visit to CN the catamaran cruise was 4PM on Monday, Thursday and Saturday. Not sure if that's changed but hopefully someone will correct me if I'm wrong. Haven't tried ziplining so can't give any help there.
The zipline canopy tour through Chukka is at the Montpelier Outpost in the hills, back toward MoBay. You actually head back most of the way on a shuttle, until you turn south in the countryside. They conduct river rafting & kayak tours, as well as the zipline tour, through this facility. It's about an hour and fifteen mins. each way (with possible stops to pick people up at other resorts), plus a couple hours for the tour itself. As the others pointed out, just sign up at the Tour Desk.