Your not alone. I searched everywhere for it. I've googled it, and followed the directions to a tee, and I was on the phone with a cast member and he was on the phone with IT and they gave him step by step directions, and same thing...the "make a payment" button was not there for me. I have had to call in my payment each time. It's not the best, but it is automated, takes a few minutes, but not as quick as an online payment I'm sure, but this is the way I have been doing it.
Out of curiosity, did you make a complete (tix) package booking or just a hotel res?