Word only does U.S.P.S. type barcodes, but if you are not using postnet
barcodes for bulk mailing discounts, the USPS also has an article saying
that

"unless you are preparing a mailing for automation rates, you should NOT put
barcodes on the mailpieces. As long as the mailing address is correct, your
mail will still be processed efficiently without a barcode-and
you won't need to worry about having the wrong barcode on your mail."

http://www.usps.com/businessmail101/misc/discountsBarcoded.htm

One of the links in the article says that you can't get an automation-rate
discount unless your mailing list has been verified by Coding Accuracy
Support System (CASS) software and you're sending at least 200 or 500 pieces
(depending on the mail class) at one time.

Part of the problem is that the standards for generating barcodes have
changed over the years and Word's feature has not kept up. Microsoft has an
article about this here::

http://support.microsoft.com/kb/897290

In other words, if you're using USPS barcodes for bulk mailing discounts,
you have to use a package other than Word. If you aren't, you don't need
them and probably shouldn't use them. And if you aren't using /U.S.P.S./
barcodes, Word doesn't do them.

If you still want to know how to use them, why not search this group in
Google groups for e.g.

Jamieson Barcode

Peter Jamieson

You can use a Barcode field with which you can display either a POSTNET bar
code, a FACING ID MARK (FIM), or BARCODE is US Zip Code, by adding the
relevant switch and incorporating the relevant field from your data source
into the Barcode field.