If you click on "My Plans", Scroll down to the first day of your trip where it shows your resort. It should show your reservation number and "Cancel Reservation" in blue. Below that should show your balance and "Make a Payment" in blue. Click "Make a Payment" and it should take you to the pay screen.
Unless you have a room only reservation and not a package - then you'll need to call.
There is no rhyme or reason to that, and I think it's annoying, but for some reason that's the way they have it set up.
Hope that helps and you can get it figured out.
Have a fun trip!!