Where's my Applescript application?
The AppleScript editor is called Script Editor.app and usually resides in /Applications/Utilities/.

If the file was deleted you may get it back by reinstalling it separately with Pacifist by extracting it from "Install OS X El Capitan.app (10.11.0) > InstallESD" and updating it with OS X El Capitan 10.11.5 Combo Update or immediately from "Install OS X El Capitan.app (10.11.5) > InstallESD".