I was wondering the same thing. I have looked several times on my reservation and there is nowhere to make an online payment.
I called and s/w someone the other night who could not find it either. He suggested I call the next day when the IT staff were working. Since my trip is in just a few day I did not bother calling back.
Both reservation that I have are Room Only... could that possibly have something to do with it? Maybe online payment is for those who have vacation packages.