i thought it isnt activated by triggers, but its just in the hard drive or whatever it was called. maybe its one of the melee functions in the trigger editor though. anyway, u can reveal anj oponent by creating an visibility modifier (visibility - create modifier) enable it and remove black mask
well actually all triggers run using JASS, else u couldnt convert them to it right? (dunno for sure) but at least u can use the melee functions in GUI to, but the revealing thing not