When you have control of Desmond, follow the tunnel until you find stairs to your left. Go up the stairs and at the top you'll find the gates to the stadium. Approach the gates and you'll talk to Rebecca about how you're getting in. Sounds like a better idea to avoid the front gate so head to the right side of the gate where the door is blocked off.
Hop the railing and go through the exit marked with the green sign. In the courtyard just beyond this door, you'll find some security guards looking around. Wait for them to walk away and hug the wall to the left. In the grass just to your left will be some brush that you can slink into and disappear. Find the couple making out by the door leading back into the building and when the guards are gone, slip out of the bushes and up those stairs. Go into the door and you'll be back inside past the checkpoint.
As you move through the building, watch out for guards that are carrying the power rods. You'll be able to see them from pretty far away because they glow a pale green color on the floor. When you see guards walking around, you need to blend in with the crowd to avoid being noticed or if you see them standing still as in at a checkpoint, you need to find another way around them.
Move through the large stadium area here and if you feel like you’ve been noticed, head towards the cheering crowds leading in to where the game is. If you push into the crowd here, you won't be noticed. Further along this area, you'll see some guards off to your left around a corner and on the right side will be some vendors and newsstands. Walk slowly through these newsstands to break your line of sight with the guards. Move a little at a time and walk into the bathroom on the right wall when you've reached the end of the newsstands.
Walk through the bathroom and find the exit on the far side. The exit will lead you back into the stadium proper. There will be more crowds to your left to join if you need to. You'll see another check point in front of some stairs ahead but to your left will be another exit sign that leads to an engineering room of some kind. Use this room to get you outside.
Outside, you'll see a half opened shutter in front of you. Slide under the shutter to enter an alley. Climb up the small wall in front of you and then up onto the shack. From the shack, turn left and jump towards the chain link fence. Keep running and you'll cross the raised platform in the distance. Turn left and climb through the window back into the stadium. Go up the stairs and follow the hall as it turns left.
Walk along this hallway but watch for the crowds of people because there are guards at the far end. You should be able to get pretty close to the hallway at the end before he turns around. Look for a group of people on the right and stand with them. Let the guard pass by and then go through the last door on the right.
This will put you in a VIP room. From here, you can climb out onto the catwalks to the left front of the room. Make your way along the catwalks and beyond the JumboTron. Follow the catwalks until they lead you to another VIP room on the far side of the stadium. Leave the room through the back and enter the hallway. Turn left and walk down the hall. You'll see the room you are looking for partially destroyed on your left.
When you get there, you'll bump into an old enemy. He'll make off down the hall and down the stairs so chase him. He will take you back down to the lower levels and past some guards. He will enter a crowded area of the stadium and start heading right. He'll run through a door that will lead back outside to where you found the first set of guards and the couple making out.
You'll get into a fight with Cross and a few guards at this point and it's going to be a little tough because you have none of your normal HUD elements. Watch for people making motions to attack and certainly watch out for when they draw their pistols. You can hit the A/X button at this point to use one of the guards as a human shield. Fight your way through these enemies and then find Cross' body to grab the relic.
With this in hand, guards will give chase. Go around the corner and back to where you were standing in line to get by the ticket checkpoint. Immediately turn left and vault over the railing, run down the stairs at the end and turn right. Watch on the right side for an exit and run through the gate to find your train.
