If the add-in is correctly installed, it adds a group to the Mailings tab of the ribbon.

Open your Letters merge document, ensure the Excel data source is attached and the document is saved. Click the Start Merge button in the add-in Group.

By default the template goes in the Word startup folder (with Word closed) If you have not changed the preferred startup folder it can be located (in English language versions of Windows) by typing
%appdata%\Microsoft\Word\Startup
in the Windows Explorer Address bar and pressing Enter.

This is all explained on the web page?

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Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com