How do I add a plus sign sprite ?

That spawns on random coordinates for each turn of the game ?

And system accepts its origin point as origin point for each turn .

Also ( thats the most hard one for me ) how is that plus sign will always fits to the all screen. Should it be infite plus sign to fit always ?

This is for your first sentence

Action:

Create object "PlusSign" X: random(ViewportLeft,ViewportRight) Y: random(ViewportTop,ViewportBottom)

did'nt understand what you meant with the two last sentences.

If you could specify what you mean I would be happy to help.