I wonder if this is the same problem that myself, and others, have encountered. Is this build error something like “This MGFX effect was built for a different platform!”? If so, try removing the /platform line in the Global Properties section of your Content.mgcb file.
If you want to get a more detailed error message, I think you might have to run that command manually. Copy/paste what it spits out in the error window and run it in the Mac equivalent of a command prompt.
Anyway, I’m just guessing here… I’m not sure if this is the same issue, but worth a shot.