Correct, the first response did have the right answer.
Yes, you can use your MB to pay for an
ECV rental in the park -- assuming you have an on-site stay and set it up for charges. However, I will suggest to bring a credit card anyway just in case. There have been times that the system goes down and nothing can be charged to MBs, at which point you either need a CC or cash to do the transaction. This is true of other in-park venues (dining, shopping) as well, not just
ECV rentals.
Enjoy your vacation!