To install gtkmm3, run the following command in macOS terminal (Applications->Utilities->Terminal)
sudo port install gtkmm3
To see what files were installed by gtkmm3, run:
port contents gtkmm3
To later upgrade gtkmm3, run:
sudo port selfupdate && sudo port upgrade gtkmm3
Reporting an issue on MacPorts Trac
The MacPorts Project uses a system called Trac to file tickets to report bugs and enhancement requests.
Though anyone may search Trac for tickets, you must have a GitHub account in order to login to Trac to create tickets.