you can use the choose operator like this:
System.Every 5 Seconds:
-> System.Create Object Enemy at layer 0 at (Choose(x1, x2, x3, x4), Choose(y1, y2, y3, y4)
You can add as many parameters to the choose operator as you want(I think). Change the x1 to x positions you want obviously.
Hope that helps! <img src="smileys/smiley1.gif" border="0" align="middle" />