If it is a transfer through your bank account then you use the bank account number.  

It is only when you pay through paypal that you have to option to use either bank account number or debit card number.

Oh, and the Mastercard debit card is not as good as the Visa debit card....they are a lot more selective with what they will chargeback.

I don't recommend anyone to rely on a Mastercard debit card at all.