Have you re-started Word since installing the Add-in?
Do you see the Merge Tools tab on the Ribbon that will appear if the Add-in has been loaded.
If you click on the Developer tab of the ribbon and then on Document Template item, in the list of Templates and Add-ins, does the MergeTools - 20140218.dotm item appear in the list of "Global templates and add-ins" and is there a checkmark in the box before it?