What do you mean by "Spawn monsters randomly"? Do you mean spawn them at random time intervals or spawn them in random locations on the map?
For random time intervals and random location you could go:
+ System: Every Random(1000)+1000 milliseconds
-> System: Create object BadGuy on layer 1 at (Random(LayoutWidth), Random(LayoutHeight))
That would spawn an enemy once every 1-2 seconds at a completely random spot on your layout.