I was given brief access to a Mac OS X 10.5.5 laptop. The procdure is similar to what I described above:
At the top left are two Icons for "default Browser" and "IP Printer". Since none of these are IP Printers, select "Default Browser". When you select "Default Browser" your printer will probably not appear.
In Mac OS X 10.5.5, there is a set of Icons along the top of the dialog box. The one with stylized left and right pointing arrows is for AppleTalk. Click on that Icon.
Within a quarter minute, any available AppleTalk printers will appear in the list. If they do not, there is a problem with cabling or the "Bridge" device, or AppleTalk is not on or not assigned to the correct Network Port. (If AppleTalk is not turned on, it will warn you about that and allow you to fix it.)
Select your printer from the list. You may now either Autoconfigure (Printer set-Up will query the printer for its identity) or Manually choose an available customization file from the list provided. You should choose the one for your printer, the LaserWriter Pro 600, not a third-party driver for a different printer.