Forum rules
Before asking on how to use a ZDoom feature, read the ZDoom wiki first. This forum is archived - please use this set of forums to ask new questions.
Action 80, script execute. (assuming you're writing a script, but the rule below works for every action special).
You can select from the line properties the activation mode. If you want to spawn the thing when you cross a line, put "Player walks over". With a switch, "Player presses use". When a monster crosses the line "Monster activates", and so on.
Ok, I had it all correct and for some reason it didn't save the setting I had on the linedef. It all works fine now though, thanks man. (sorry for the dumb question haha)