HP Deskjet 5850 Color Inkjet Printer - Manual (Page 128)

Do not discard the Rear Access Door. In order to print, the
duplexer or the Rear Access Door must be attached to the
printer.
2. Insert the duplexer in the back of the printer until both sides snap into place.
Do not press the buttons on either side of the duplexer when
installing it in the printer. Use the buttons only when
removing the duplexer from the printer.
printing with the duplexer
For instructions on printing a two-sided document with the duplexer, click here .
For instructions on making automatic two-sided printing the default print setting for
your printer, click
here .
supported media types
The duplexer only works with certain media types. For a list of supported media, see
the specifications in the documentation that came with the duplexer.