I am trying to place collision markers in my castle mod I am building, but when I go to create a collision plane, nothing happens. It makes it so that I can't select any object in the render window or anything in any other window. To be able to do that I have to click the [C] button again. The [C] button looks like it has been pushed in when I've clicked it, and I do have markers toggled visible. Anyone know how to fix this problem?

Try selecting a static piece first then press the collision button.