All you do is click on the event and you choose 1 of 3 matches to play. Its located in the online battle mode, but the matches are you vs an AI, and it plays out exactly like any other free-battle (at least for now). This specific event's theme is the Akatsuki capturing the Bijuu, so you'll be forced to play as one of 4 Akatsuki teams against AI Jinchuuriki. There are 3 separate fights to choose from, each with 3 consecutive battles within them - and each separate fight increases in difficulty.
I played the hardest set of battles, and like I said its the exact same as free-battle, only you are forced to play as Akatsuki members. So you're going to have a hard time if none of the members you regularly play as. Other than that, this event isn't anything to write home about, but hopefully future events are more, well eventful.