I can’t comment at all about running on Mac OSX, but I did myself experience some time in the past a version of eddy that I didn’t observe using more than one thread. I suspect this may have been the version of eddy provided with FSL before it was moved to closed source. Therefore I would suggest at the very least acquiring the most recent stand-alone eddy_openmp binary. However we really can’t provide assistance on an FSL command beyond that.