After you use your new card, you will get a notice regarding the DVC maintenance fee. That's what happened to us at least. It doesn't happen right away.
You will need to use the card once and then they will send you the notice. I got mine about 10 days after using the card for the first time. The coupon says that your DVC account has a note on it too take the $75 off and you don' t need to send the coupon in. I will probaby call MS around the New Year to double check.