you must be confused, there is no delta membership reward card.
There is a delta skymiles card (gold or platinum) that earns skymiles which are directly deposited into your account. Then there are a few kinds of Membership Reward cards which earn Amex points that can be transferred into many other programs. As long as you are on their e-mail list, they will notify you of current bonus offers like the 50% transfer to delta bonus. Also you can check the delta home page under skymiles/how to earn for a list of all the current partner offers. When Amex offers a % bonus for transfer into Delta skymiles it usually lasts for a couple of months, so unless you are totally snoozing you shouldn't miss out on it.