This has probably been covered else where, but if you have an Epson R300 or similiar model printer and getting the "CD / DVD tray not set correctly" message ... try resetting everything and watch the back of the printer when it first pulls in the tray. It might be hitting what ever is behind the printer. Yep the tray actually projects out the back of the machine when it first pulls the tray into position.

If that isn't the problem then the drive rollers probably needs cleaned and/or the bottom of the tray needs scuffed up a bit with sand paper. Be sure to wash / dry the tray if you do any light sanding.