we picked ours up at guest services...just cross the lobby and stop in.
note: while you might not use it much on board, if you get it first thing, then i can hold all your carry-on luggage until you get to your room. also, they are the jogging strollers, so they are big. we found that we had little space to store it in our stateroom, so we just tucked it into one of the spaces created by an inside door to the ship's interior (we've only had one on deck 8). if it was an inconvenience to any of the stewards or other cms, we never heard about it. i've also seen someone store theirs in a space created across from the elevators.