Install the ffms2 library which is not present in MacPorts. Download it here: https://github.com/FFMS/ffms2, in the ffms2 folder, type from the terminal:
./configure --prefix=/opt/local make sudo make install
Download the SIRIL sources:
svn co https://free-astro.org/svn/siril/branches/0.9/ siril
In the terminal go to the siril folder:
cd siril
Generate the configure file by typing:
./autogen.sh make
To install Siril, use the usual:
sudo make install
Launch Siril:
siril
Enjoy!