Ok this is goinna be the joke of the day but when you get a terrain in the viewport and you have a tool like noise selected and the red circle is on the terrain, what button do you push to make it work?
Ctrl+left mouse button and then just paint the geometry (right click to do the opposite).. I know it annoyed me too when i started i just watched some Youtube vids, they were using a different version of the engine but the hot keys seem to be the same =D. Hope I helped. (im pretty sure the guy above me is wrong)