Insert a frame, using the button on the Forms toolbar (looks like a US
flag). Drag the frame edges to the desired size. Insert a second
paragraph mark below the initial one.

Format the first paragraph with a left indent or tabstop to position
the FIM. Format the second paragraph with the desired left indent for
the address.

Insert the return address in the second and subsequent paragraphs.
Select the address (actually, just the line including the zip code is
enough) and insert a bookmark. The choice of name is up to you; RetAdr
seems appropriate.

In the next paragraph below the address, go to Insert > Field, select
Barcode, select the bookmark name, and check Postnet and, if
appropriate, US zip code. Click OK. You may want to add Space Before
to the paragraph formatting to move the barcode away from the address.

Put the cursor up in the top paragraph of the frame and repeat the
Barcode field entry, but check the FIM box and enter either A or C for
a courtesy or business mark, respectively.

You should consult USPS publications such as
http://pe.usps.gov/text/Pub25/Pub25.htm and
http://www.usps.com/cpim/ftp/pubs/pub177/welcome.htm to be sure your
positioning is correct.

--
Regards,
Jay Freedman
Microsoft Word MVP         FAQ: http://word.mvps.org