You need to add the Custom Document Property named "InvNum" to the document, as discussed in post #2 of the thread in the link (
https://www.msofficeforums.com/word/...html#post19265)
You will also need to add the DOCPROPERTY field for the numbering to your file, as per the Invoice.dot demo.
Alternatively, you could have simply edited the Invoice.dot demo by adding your own content to it.
Finally, you should also be saving the Word file as a Macro-Enabled Template, not as a Document.