Are you saying that you want a marker at the edge of the screen that will point the player in the correct direction to move in order to find the offscreen actor?
If so, you need the following:
- The X,Y of the offscreen actor
- An imaginary line that connects the player to this actor
- the intersection of that imaginary line and the edge of the screen
I don't know the precise solution, but it seems like raycasting would likely get the job done, and I believe there's an extension for that. Maybe there is a simpler way ... ?