You beat me! From past experience, they put the certificates in your account. When you make a reservation using points, if you have a cert that qualifies, you are asked if you want to use it. Say yes and that is all there is to it.
One good thing I noticed about the certificates is that the expiration date is a year instead of the usual 6 months on others. Whoopee! I hope the year time frame is a new standard for all the free night cert's!