Did you use the lookup link near the top of the first page first for the existing CID? that's where i was goofing it up (had to call them)
I saw someone else said to use the parent's DL, but I had problems with that on my own account on Reserve America.
P.S. the guy on support said that soon (don't recall the date) you won't be able to reserve CO campgrounds on Reserve America directly but will have to go through the CPW site (though they use the same back end).