The promotion gives you $50 of onboard credit for using a MasterCard for your Sail & Sign Account (onboard purchases).
1)You must sign up by November 15, 2012. (see link below to sign up)
2)You must sail on a Carnival ship between the dates of December 1, 2012 and March 15, 2013.
3)You can only receive one $50 promotion per cabin.
You must having a booking number before signing up:
Sign Up For MasterCard Promotion
Credit should show on your account in about 3-4 weeks.
If the credit does not show before embarking, print out the confirmation email you receive when signing up and take it with you.