Hi

We were playing halo 3 on some xbox 360 today. My ideal game basically just involves guns and lots of aliens, so we thought it would be good to try campaign mode.

Just started the very first mission, running through the woods following the team after having reviewed the controls (which we all had set in Default Mode) and after a minute got to this bit with a waterfall, at which point all the AI characters started scaling up this rock face next to the waterfall. (know the bit I mean?)

For the life of me none of us non-gamers could work out what you have to press in order to climb! Tried just looking up and trying to move up the wall and also jumping up the rock face, also pressing and holding the action key/right reload button whilst doing that, etc.
Eventually gave up after about half an hour feeling somewhat vexed.

Just exactly what do you have to press in order to climb?

Haven't played it, so I don't know how much help this will be: The Halopedia

I tried a search but you'll know better than I the terms you need to enter.

Only the NPCs have the ability to scale/climb these walls -- players, however, don't have this ability.

Look around for a 'ramp' or a passage way that leads up to that place (been several months since I've played Halo 3)