Currently there are three different sets of mail merge templates, under CRM, Fundraising, and Events. There only needs to be one. My outreach director and I just wasted a good bit of time on this point which would have been saved if all the templates were in one location.
There is a 4th mail merge template area under Email Marketing. Not sure if it is shared with the other.
I am struggling with this same issue the past few hours. I set up a campaign to send ticket numbers to event registrants and I can't select the ticket number mail merge field from the campaign window.
I tried to have a go at it through the event mail merge, which does not accept html from the mail merge template I set up under Campaigns. At the very least, set it up so it accepts html so we can manually share the templates from one area to the other.