OK let's try use the web version of
MDE to access your profile and add up to 6 payment methods. Yes MDE still has some web functions to use. The one you want to use is your Account Profile:
https://disneyworld.disney.go.com/profile
You will need to enter your MDE account password even if you Name is displayed on the top Blue bar. If you are not already Signed In to your Account then you will need your Account Name an Password.
If you don't want to use the direct link just go to any
https://disneyworld.disney.go.com/ page on on top bar you can Sign In and then Select the
MyDisneyExperience link on the top navigation bar on the right side. It is right next to the Cart Icon.
Select your Profile, it is the first link with your Name and your Character Image. On the Profile page you just scroll down to the Payment Method and click on the line. You will a place to add a Payment Method.
Once you have made your additions / updates to your Payment Methods. You can just close your MDE App and reopen it and your Payment Options should be available to use.
Dave