Go to the mission "Halo" and select mission start.

Go to where you would find the terminal (when you are in the cave before you fight the flood you will see the cave turn left and see bars sticking out of the wall,

Go right and you should hear Cortana say "Where are you going", then instead of going to the terminal, turn right and you should see an open space with a bar.

Then use the fuel rod gun and have Master Chief grenade jump, while the second player shoots him with the fuel rod gun before he falls (that is the hardest part to do, I tried a lot before getting to it).

Once on the platform look to your left.

You should see a man jumping up and down with a towel wrapped around his waist.

If you can't see him zoom in.