From what I can tell from your macro code, the start/stop asterisks are not included in the data source. In that case, you need a field coded as:
{QUOTE "*{MERGEFIELD barcode}*" \* Charformat}
or:
{QUOTE "*«barcode»*" \* Charformat}
where 'barcode' is you mailmerge field's name and with just the 'Q' in 'QUOTE' formatted in the 3of9 barcode font & point size that you require.

Note: The field brace pairs (i.e. '{ }') for the above example are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac); you can't simply type them or copy & paste them from this message. Nor is it practicable to add them via any of the standard Word dialogues. Likewise, you can't type or copy & paste the chevrons (i.e. '« »') - they're part of the actual mergefields, which you can insert from the mailmerge toolbar.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]