As a work-around, download the MergeTools – 20140218 Add-in that I created from the following page of my One Drive:
https://onedrive.live.com/?cid=5aedcb43615e886b#cid=5AEDCB43615E886B&id=5AEDCB43615E886B%21566
Save the file in the Word Startup folder.
In Windows Vista and Windows 7, 8 or 8.1 the default location for that folder is
C:\Users[User Name]\AppData\Roaming\Microsoft\Word\STARTUP
When you have done that, the tab shown below will be added to the Ribbon:
One thing to note is that the field names in your data source must contain only alphanumeric characters (No @,#,$,%,&,(,), etc) and the field names must not start with a numeric character (0-9).
You may also want to download the Merging with Attachments document that is also on that page which explains how the system is used.
It is not actually necessary to have separate attachments as the facility can be used to send just the documents created by the merge itself as attachments, either in the form of Word files or .pdf files.