For this achievement you will want to load up the map Beaver Creek on Halo 2: Classic.

You will want to start by going up the ledge where the sniper is located.

From there you will see a few boulders on the ledge, jump up there.

Use a grenade to move the boulders, and you will see a note in the rock saying "Why Am I Here? - Not Sid".

Look at it for a few seconds to unlock the achievement!