Adding explicit instructions about /var/lock

This cost me at least 2 hours of wrestling until I stumbled upon this thread:
https://github.com/jantje/arduino-eclipse-plugin/issues/129

I have a 64-bit version of jnlb serial library, but I was still not able to write to serial port under /var/lock was created. So  I am adding a check for for MacOS and /var/lock being accessible so that nobody has to waste anymore time :) 

If there are alternative means of removing this unfortunate stumbling block, feel free to reject :)