How do I set up Unison to open the meld GUI when I choose to merge in the Unison GUI
Here's the documentation for the merge option. The syntax for merge is
merge = <PATHSPEC> -> <MERGECMD>
Where <PATHSPEC> is which files you want merged with <MERGECMD>. So the line
merge = Name * -> meld CURRENT1 CURRENT2
is telling Unison to merge all files that match the path specification Name * (which is all files) and merge them with the command meld CURRENT1 CURRENT2. The meaning of CURRENT1 and CURRENT2 is explained in the documentation.