There should be a continous channel/slot/groove (Or whatever you may want to call it) that runs around the entire perimeter of the window on one side. (On most windows it would be on the interior side)
It is refered to as a "screw race" or "screw boss". It is wide enough for the shank of the screw to fit into, but narrow enough for the threads of the screw to catch onto each wall of the opening of the race or boss...
(It will appear as a narrow dado groove)
On some makes of windows it also doubles as a groove to hold the vinyl screen spline, which holds the screen material secure.
The predrilled holes in your ring should line up with it. Once you get it started it will be easy to tell if you missed hitting the race. I would leave your screws a bit loose, until you have all the screw propperly inserted into the race before tightening them up.
Hope this helps...