Take the barrels under water and put them into the cage with the other one, you'll know when you see. The ramp will then be jumpable. I hope this is where you are or I just spoilt a piece of the game for you.
I feel for you mate, it seems you DVD may be courrupted as LittleB said above, get it back to the store asap and switch it, and try to have a merry Christmas!
I don't actually look at my keyboard at all during HL2, I know where every key is, as stated maybe that is due to knowledge of your keyboard and as you said, your lack of HL experience. If I started playing Halo 2 now, without playing Halo in my life, I would be looking at my keyboard as well...