You have to talk to Ian about the nest. He will ask you to get him a gearwheel, which is located in the old mine, near some minecrawlers. You bring that to him, and he will tell you how to get the gateguard to open it up.

hiitzme, you are correct about that, but he's problem is about the entrance of the old mine, not the nest. There's a bug, if you use the switch near the mines entrance to close it, it has a chance to be bugged in. So, if the entrance of the old mine is closed, and you can't open it with the switch next to it. Then you have to start a new game, or load a saved game, where you havent used the swicth.

English isn't my native language, so i hope i was clear.

Actually, you don't have to re-load if you hit the bug(from what I read) ; in the faq's, they describe how to "cheat" your way through the bug.