Can I process debit cards through MM+ Select?

Yes.

We do offer Debit card processing for signature Debit transactions instead of PIN Debit transactions, meaning the customer will sign for the purchase instead of entering the PIN Number. 

You will process Debit cards by using the Credit Card Purchase option in the Transactions menu. 

If the card you are trying to process is not branded, in other words, if it does not have Visa, Mastercard, Discover, or Amex on the front of it, you cannot process it, as it is not supported by MM+ Select.