Hi Remi,
I noticed a typo in the commands you used. "selfupdate" should be one word, not two. Perhaps this was just a typo in your email, but if not then you may want to try running it again. Also, I noticed that the instructions at the Demeter website say to log out and log back in after installation, so you may want to try that if you have not.
If that is not the problem, here is the sequence I used to get Demeter working on my system:
1. install XQuartz 2.7.7 (xorg-server 1.15.2).
2. install Xcode
3. install command line tools by typing "xcode-select --install" in the terminal.
4. install macports:
https://distfiles.macports.org/MacPorts/MacPorts-2.3.3-10.10-Yosemite.pkg5. ensure macports is working by typing "sudo port -v selfupdate" in the terminal.
6. run "sudo port upgrade outdated".
7. install Demeter using the command "sudo port install demeter-devel".
8. log out and log back in (I didn't need to do this, but the instructions say that it may be necessary in some cases).
George