To unlock the terrorist campaign, simply beat the first campaign all the way through on any difficulty setting.
To get in a rock (on Srilanka only) face one of the corners (where it juts out with a point, not like some small turn) and press CAPS LOCK and F. You should now be crouching and walking. Now if you're right up close to the rock face back up. Now while still crouching and walking begin moving towards the corner you chose. When you near it jump towards it and as soon as you touch the rock face press F8 to take a screenshot. If it worked right, you should be in, if not, just try again. Using this you can get in the rock on the upper waterfall that's right next to the tower bunker and snipe any Army players without them knowing. You can also get in the rock on the upper river (!!) and that huge one next to the army spawn that seperates the sandy part from the water (this one may be the most valuable, bust in with an AW50 and when the helicopter comes to the spawn let it go).
To get on top of the boxes is very simple. There are 3 groups of boxes you can squish yourself into, one is the group that faces the PSI entrance to the warehouse (this is an EXCELLENT method of racking up your kills, grab a silencer and hit it), another is the group in the right side of the warehouse (if you came in from Krongen's entrance) where the indent is and you can go up the stairs (left side of the warehouse if you came in from PSI's entrance), the last is the group in the left of the warehouse (for Krongen--right for PSI). To get in each one it's basically the same thing, just jump onto the smaller box that is already there, press CAPS LOCK and F (so you walk and crouch), and jump forward. This is almost impossible to fail unless you messed up pressing one of the keys like you were already walking and then you unwalked or you didn't jump FORWARD...however, that one in the light (on the left side for Krongen--right for PSI) is an exception. You have to jump onto the corner that sticks out on the left side (if you're facing it from the main path of the warehouse). It may seem to small to get on, but you actually can, and from there repeat the steps (walk, crouch, jump). Now head up and try it!
This is the roof near the VLF Temple spawn, right out in the Temple Graveyard. Jump in that opening in the wall (the one on the left--not one of the regular paths with stairs). Now jump on the first pillar in front of you. If you don't make it, try again. When you get it, jump to the second pillar. When you get that, jump to the third pillar. Now, if you miss this, you have to start all over, so it get it right the first time. Jump on to the wall in front of you. Now get on to the other wall to your lower left and walk along to the roof. From here, you can go anywhere on the roof. If you step on the place that looks like it has no ground, don't worry, you won't fall through. And they can't see you. You can get a gun and hide up there if the Army is pressing down on VLF.
To get over the Krongen fence (I'm not quite sure with PSI) in the south yard go over to that mound of snow that sticks out. Face the middle corner (the one that juts out into the yard the most) and press CAPS LOCK and F. If you're right close up to the mound face back up. Now move toward the mound and when you're near it jump forward. When you hit the mound face from your jump press F8 to take a screenshot (which pauses it slightly and confuses the game). If you did it correctly, you should be on top of the mound, if not, just try again. From there you can either fall back behind the fence or walk along the top of the fence to the other mound next to the warehouse. A good strategy with this is when you're on PSI and your team is pressing down on the Krongen base you can go back to where the building would be and see through the wall and see anyone walking around inside! You can even shoot them too!
The only thing that allows this is lag, so it won't work in single player and works better with higher pings or a server with a lot of people. When you enter a server at first you can do this with any specialty except sniper and I think demoman (because the amounts between the weapons are too much). This description is with heavy gunner. Buy the first weapon and then VERY quickly go over and buy heavy armor (you don't actually have enough money, but lag will cause a slight pause after you buy the gun giving you enough time to get the armor). If it doesn't work at first sell the gun and try again. When you get it, sell the gun and click your armor back down to medium, and BOOM! You have more money than you started out with! Now repeat, except get the second gun. After that you should have enough money to get the third one, and then the fourth. Now when you get the fourth gun and the heavy armor, you should have around $7100. If you sell the gun and armor, you'd get about $7900. I always stop at that amount and get my gun or whatever for the sake of time, but you can actually go up to $13000 or something if you do the shotguns and stuff. You can do this as long as you have $300, which is enough to get the weakest pistol.