For some reason they make this really hard to find. On mine, if I scroll all the way down my plans on the last day in my Disney experience it shows my balance and in teeny tiny print just above it says make a payment? I’m surprised Disney did such a poor job with this,.,they are usually so much better at asking for money...
If you have a room only reservation you can't pay online. You have to call to make the payment (it's an automated system, so usually fairly quick and easy since you don't have to wait to speak to CM).