The cast member ringing you up will usually say something to the effect to let you know you are receiving it. It is reflected on your account. If they do not, it does not hurt to ask.
I always ask to make sure they added it. A cast member told me that a very small "gold" or "platinum" icon shows up on the register but that it is easy to miss.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!