Animmaniac's solution should have worked. It appears to be a bug that it doesn't work with "fullscreen on(scaled)" as it works with "fullscreen off" and "fullscreen on (crop mode)". To be exact the scrollx and scrolly expressions are returning a position that's in the center of the screen but doesn't seem to have a correlation to the screen size.
Now for a workaround select "fullscreen on (crop mode)" and set the scale manually. Then use these actions:
Set layout scale to: min(WindowWidth/640,windowheight/480)
Sprite set x: scrollX - WindowWidth/(LayoutScale*2)
Sprite set y: scrollY - WindowHeight/(LayoutScale*2)