Cruise was nice/ questionable

I booked my cruise back in November over the phone and was told that among the amenities I would be receiving was a $100 on board credit. When I went to check while on board to verify this, I was told there wasn't a credit.  Upon returning to my cabin, I noticed at the bottom of page 2 of my receipt under details was a statement that said I needed to apply online for the credit 21 days before departure...  I have booked with other cruise sites (smartcruiser.com) and didn't have to do this-it was automatic  Neither was I told that I needed to do this.  It seems that the offer of an on board credit is in questionable "good faith" on the behalf of .  I then emailed on their website with my complaint, to which was stated that emails would be replied to within 24 hours.... that was 3 days ago....