While you always use Facebook to login to MFP, they do let you setup a password as well so that you can login from Waterlogged—even if you never use it. To setup a password in MFP:
- Go to MyFitnessPal
- Your username is shown in the top area as “Hi, username”. Write that down.
- Click on Settings, over to the right of you username.
- Click Change Password at the top of the list on the left hand side.
- Set your password.
That should get you a MyFitnessPal password you can use in Waterlogged! If that doesn't work, contact support and we'll see what the next steps could be for you.