I'm baffled that this solution has yet to appear in this posting. I had this same exact problem and attempted to following the recommended steps and even downloaded the recommended add on. Ultimately what fixed this for me (and I have a mail merge going
as we speak) was to repair/fix office 2013.
Steps:
Go to control panel
Select programs
select programs and features
select on Office 2013
choose the "change" option above the listed programs
select "repair"
This will repair any errors with permissions/installation. Once this completed my problem was fixed.
I hope this helps others out there!
Cheers