We assume that people who use FSX and Weapons know how to locate aircraft but since I had trouble with the load as Admin, I don't assume anything.  Of course we are only shooting down commercial Aircraft for fun, or we received an "executive decision".

If you want to use the F-18 Radar to locate and shoot down AI Aircraft see this excellent tutorial by http://fsxblueangels.com at http://www.youtube.com/watch?v=9tfBFysVCg8&feature=player_embedded

They have awesome videos and other tutorials on the radios on youtube and pdf.  Enjoy