Hi, i am starting back up to build Custom Zombies maps and am gonna have a butt-ton of questions. I need someone who is gonna be able to help me! Here is my first question.

when i try to open radiant, a box pops up that basically wants me to open radiant with something. i dont know what to open it with. so when i exit out of that box it says "unable to load project file path"

ANY HELP?????