As I mentioned in another post, just keep trying. If you are a 1st time DCL Cruiser, you can get the $50 credit. Just tell them it's the REF code.
I tried several times, each time getting a different DCL operator. After several different versions of "no", I called and got someone who applied it - no questions asked. I know it doesn't make any sense, but it really depends on the operator you get.
WDWO, I am also going on an April cruise that's full and just got my $50 credit added on Friday. Just keep trying until you get a "yes". That seems to be what everyone else is doing, and it works.
Good luck!!!