So, the mail merge add on doesn't actually appear in my add ons at the top bar (like my avery label merge does). That said, I did get further today using the side bar (I was going from docs and selecting the sheet). The files appeared for me to choose from, however, after selecting the file, it said -
Something went wrong when executing the add-on.
Then, I opened the sheet file and attempted to select a doc to merge and it just kept going back to select a document...I didn't get anywhere.
Thinking it was the add on, I uninstalled the "Mail Merge Documents" add on and installed "Mail Merge" and so far this one works a heck of a lot better.
Bummer though - I use googles platform to avoid paying for the Microsoft suite, but the Mail Merge add on is only free for 30 days....I wish GOOGLE would add this feature to their platform with no charge...