In order for the LaserWriter Pro 600/630 or the LaserWriter 16/600 PS printers to print to the envelope feeder, you have to cycle the power on the printer after installing the envelope feeder.
Check the startup test page and verify that it indicates that the envelope feeder is installed. If not, power off the printer, reseat the connection to the feeder, and power it back up. Once the printer recognizes the feeder, you can use the Auto Configure to list the feeder as installed.