It doesn't make sense that you are able to use macros, but not install an Add-in. However, if you download a copy of my
MergeTools – 20150422.dotm Add-in
from the following page of my One Drive:
http://bit.ly/1hduSCB
and save it somewhere on your local drive and the select the file and use Ctrl+c to copy it to the clipboard and then in Word, go to File>Options>Advanced and scroll down to the General section of the dialog a then click on File Locations and select the
Startup item and the click on Modify and then right click in the right hand side of the Modify Location dialog and then select Paste, the add-in will be installed in the correct location so that the next time that you start Word, a Merge Tools tab will be
added to the ribbon.
If you use the Merge with Attachments facility on that tab when you have a Letter type mail merge main document with an Excel or Access data source attached to it, you will be able to merge that document to email and select a field from the data source to
supply the subject for the email message with the option of having some fixed text before or/and after the text that is supplied by that field.
The field names in the data source must contain only alphanumeric characters (No @,#,$,%,&,(,), etc) and the field names must not start with a numeric character (0-9). The number of characters
in the field names, including spaces, must not be more than 40.