The buff is named WHEE! and increases experience and reputation gains by 10%. It's very fast to build up, and you don't actually have to ride the carousel itself to gain the buff, just be near to it. If you want to stand on it but not mount one of the carousel mounts, that's just fine too.
To access the Carousel you'll need at least one Darkmoon Ride Ticket in your inventory. Fortunately, these can be bought 5 at a time in books from Kae Ti, the NPC in the tent through which you enter the Carousel. If you have a Darkmoon Ride Ticket in your inventory already, one will simply be subtracted from your total as you walk into the Carousel area.
Don't confuse the ride tickets with Darkmoon Prize Tickets, which have an almost identical icon, but are rewards rather than access tokens. The Prize Tickets are a currency and appear with your Valor points and the like, while the Ride Tickets stack in your bag.
If you get onto one of the carousel's mounts, and can't seem to get off, type /leavevehicle into chat. You can also keybind this function via the WoW Key Bindings menu.
Mists of Pandaria is here! The level cap has been raised to 90, many players have returned to Azeroth, and pet battles are taking the world by storm. Keep an eye out for all of the latest news, and check out our comprehensive guide to Mists of Pandaria for everything you'll ever need to know.