{app} is where game folder is.
If your game folder is D:\Game, then {app} = D:\Game.
In INNO it looks like this:
Code:
Root: HKLM; SubKey: SOFTWARE\Microsoft\Microsoft Games\Age of Empires 3\1.0; ValueType: string; ValueName: SetupPath; ValueData: {app}; Flags: uninsdeletevalue uninsdeletekeyifempty