You put in your search criteria under awards and upgrades and check the box "coach - saver awards" (go ahead and check both boxes for X and XN).
Once you do a search for your dates and find some specific flights you then need to go to the far right of the screen and you will see 3 icons. The icon you want is at the top of the 3 and looks like a ! inside a box.
Once you click that you can set you alert.
You can set many alerts for different routes/flights (I forgot how many but something like 18 different flights).
When inventory opens up you will get a e-mail, which is really nice!