Note: Please save your game and read through the entire process before attempting this cheat.

I found this cheat out when i was just messing around in the arena lobby, here's what you do.
First go into the arena and then once you go through the archway into the lobby part then turn left toward the wall you will notice that if you hit the wall at the right angle (standing between the bench and the archway you came in from) that you will be able to see through the wall.
Now look to your left at the archway if any of you play Halo and are good at it you will see that there might be a way to get ontop of the archway. You need to have about normal athletics to do this but it may take a couple tries.
Once you've turned left and are facing it take a bit of a running jump at it and just keep pressing Y, you will then be above the door with your head through the roof. Now that you are here you can either jump into the blackness beside you and watch the level fall away(note:you respawn near where you fell from - remember that for later)or another option is to jump onto where the wall would be where the people who set your matches up.
Walk along the wall and into the room with the restoration well there will be an invisible wall stopping you from dropping down but you remember what i said before about your respawn near where you fell so simply walk over to the edge of the wall that leads up to the arena and jump off.
Now you are in the locked room and can go through the door to the arena. Insted of going into the room with the well in it you can go over to the pig pen and drop off the map near it and you will be in there. But you can't get out once you're in so its good for kicks but save before.
Now asuming you chose to go to the arena you will walk up and notice there is a gate there like always before the matches, well you can get past this gate even when you're doing a match. You can get past it before it goes down and be waiting for your enemy - what you do is turn right and try to jump on the archway just like before, you will fall but make sure when your falling to make your character move in the air toward the center of the ring and you will respawn in the center of the rin. But if you're not doing this in a match you can't get out so save before.

PS: The secret room is behind the girl who sets up your matches next to the stairs. So just jump over and you will be behind here in a room.