Hi, Joseph!
Because I did not know the answer to your question, I went and looked through my personal reservations on My Disney Experience, and I could not find my balance for any of my reservations, either. All of my reservations are "room only" so perhaps if you have a package booked, the balance will show and you can
make payments. However in a room only situation, the only dollar amount I could find was the total cost of my stay if I clicked on "Change Reservation."
So what to do? If you need to know your balance due, have your reservation number handy and call (407)W-DISNEY. The cast members will be able to tell you your balance as well as your final payment date, which is usually 45 days prior to your arrival if you have booked a vacation package. If you have a room only reservation, the balance of your reservation is due at check-in.
Check your My Disney Experience - My Reservations to see what you can find, otherwise, make that phone call. Good luck!
Cathy