This happened on our last cruise - didn't know why the OBC went up pre-cruise and wasn't going to look a gift horse in the mouth before we boarded.
Once onboard, between Guest Services and a few emails to my TA, I was able to get it all sorted out. We were accustomed to refundable OBC from the TA, but had no trouble "spending" the NR from their promotion.
If it shows up in your Planner, you should be able to spend it pre-cruise.