Set the recordset to filter the membershipNumber column on the coupon code that is entered, use an advanced recordset to add the other criteria for the membershipStatus.
use the set session value behavior to save the membershipNumber returned from the recordset. set the trigger to Recordsset is not empty.
then use the session in the discount rules.
if you need help configuring this, we can help through a premiere support appointment:
http://www.webassist.com/premier_request.php