Dear all, 

Thank you for your feedbacks. 

After several attempts, I was finally successful with the installation of Demeter. I attach a log file summarizing the different steps I experienced (and a screenshot of the main window). I guess step 2 and 5 would have been enough: 
xcode-select --install
sudo port install demeter

Indeed, several hours were required for the whole installation procedure (more than 9 hours, but my internet connection is quite slow at home...). 

I can now execute athena from the terminal, and apparently it works fine. However, I observe a strange behavior : After athena is running, I must click once on the terminal window before the main menus are active in Demeter (File, Group, Energy, etc.). But when we know that, it’s ok. 

And the most important, I don’t have any graphical window to display the spectra. Would you have any suggestion? (Xquartz and Aquaterm are already installed on my computer). 

Thank you again for your help! 
Manuel