I take it you're referring to my Microsoft Word Catalogue/Directory Mailmerge Tutorial:
https://www.msofficeforums.com/mail-...-tutorial.html
If so, all you need to do to use the mailmerge main document and datasource that comes with it is to copy the relevant fields from the tutorial document into the mailmerge main document. This is all explained in the tutorial's introduction, on pages 1 & 2.
You can't use a {MERGESEQ} field on its own. Indeed, if that's the only field in your document, you can't even complete a mailmerge. If you document has other moergefields, though, a {MERGESEQ} field will return the Sequence # of each merged record.