For those working on Dot NET framework on OS X (El Capitan version 10.11) with mono, you may encounter this issue
A quick work-around would be to configure Mathematica 10.3 with the absolute path to where you have installed mono binary:
In[3]:= Options[InstallNET]
Out[3]= {Force32Bit->False,MonoPath->mono}
In my case I use the OS X installer and the default location is /usr/local/bin/mono. I change the MonoPath to this location in the InstallNET function:
