It does sound like a runaround. Believe it or not, if you call Carnival to book a cruise that person does get a commission. You do know that commissions don't affect the price you pay for the cruise - whether it is done online or by phone with Carnival, or with an agent?
Perhaps you could cancel the booking and then do it again with the veterans discount? There can be rules about canceling and then rebooking on the same cruise, so I am just speculating. I think Carnival has to have a copy of your DD214 on hand first. We have many Carnival experts here who should be along soon to give you the straight scoop.