hodgest (and all):
You can redeem your e-certificate online. The e-certificates should be showing on your Activity Page on marriott.com.
Book your reservation using Marriott Rewards Points. The system will tell you how many points are required for this stay per night. When you get to the part where you process the stay (I think the third screen) a block is shown (that is already checked) that says you have an e-certificate you can use for this stay. When you complete the reservation it shows e-certificate and 0 points.
Hope this helps!