What currently works for me is to install macports.  Then:

echo rsync://build.openmodelica.org/macports/ >> /opt/local/etc/macports/sources.conf
sudo port selfupdate
sudo port install openmodelica-devel +libraries

The instructions don't mention that you need to run with "sudo". 
Also, after the install there is a bug in the port file and it builds a component of OM that it doesn't correctly install.  So, Macports then complains that there is a linking error and tries to rebuild a few times before it finally gives up. 
But, best I can tell, OM does then work fine (except, I assume, for that one mysterious component that is not being linked properly)