Ah, this then?
Graphics 800, 600, 0, 2
SetBuffer BackBuffer()
px = 400
py = 300
Repeat
Cls
dirX = (MouseX() - 400)
dirY = (MouseY() - 300)
If Abs(dirX) > Abs(dirY) Then dirY = 0 Else dirX = 0
MoveMouse 400, 300
px = px + Sgn(dirX)
py = py + Sgn(dirY)
Oval px, py, 10, 10
Flip
Until KeyHit(1)