It sounds to me like you're specifying a paper size or type that is not configured in any of your trays. If you're being asked to "Load Tray 1 Letter Plain" (for example), then that is what's specified in the driver or possibly the application.
In this example, if you have Letter size paper in Tray 2, make sure you have specified the Paper Type as Plain. If there's a match between what's specified in the driver and on the printer, the paper will be pulled from that tray automatically.
If you don't care about paper type, make sure you have the driver set to use a paper type of "unspecified".