I can't seem to do it. Whenever I pick up the box with the girders, my worm switches to Girder-building mode. Which means that he can't walk anymore, and he's limited in where he can put the things. I don't see any way how he can use the girders to reach the top of the lighthouse if he can't put the girders within jumping distance of the lighthouse.
Am I doing something wrong? Help!

The solution is very simple. I just finished the mission today. You have to press triangle. This will cancel the "girder building" mode. then climb up to the overhang that is over the door under the light house. Press up or down on the pad to change the angle of the girder. Make it biger with square. When it is at a descent angle deploy it. same for the other one. When it says to find a good place to hide for a earthquake, go to the ship, as a crate will apear there. This crate will help you get to the one on the billboard. There you go.

quote Moose Man

The solution is very simple. I just finished the mission today. You have to press triangle. This will cancel the "girder building" mode. then climb up to the overhang that is over the door under the light house. Press up or down on the pad to change the angle of the girder. Make it biger with square. When it is at a descent angle deploy it. same for the other one. When it says to find a good place to hide for a earthquake, go to the ship, as a crate will apear there. This crate will help you get to the one on the billboard. There you go.

There is no TRIANGLE on the Mac, what then?

'alion

Just press backspace.
Then the girder will go back in your inventory.