You need to go to a state= to show the hud, probably go to another state after to unshow it after performing its function..since I have no idea what its doing I have left out the actions but for a simple example:
;Artificial Intelligence Script
;Header
desc = PopUpMenu
;Triggers
:state=0:hudreset,hudx=50,hudy=45,hudimagefine=entitybank\AssetsForNightmareFunhouse\HUD\MainMenu\EnterTheStory.jpg,hudmake=button,hudtype=10,hudname=enter
:state=0:state=1
:state=1:hudshow=enter,state=2
:state=2,scancodekeypressed=?:hudunshow=enter
;End of Script
I cant offhand remember what conditions you might require, perhaps hudselectionmade=, to use a mouse click for your button or if a simple keypress will do for you but this will give you a start.