I've updated the mixer to work independently of Electrum mainline now. As long as you have the latest version you can now simply drop the mixer.py file into your plugins folder and it will be available and can be enabled. It no longer needs a separate fork or Electrum version.

The standalone mixer plugin file is now:

https://github.com/tkhaew/electrum/blob/mixer_plugin/plugins/mixer.py

The default folder where this is can be dropped (on my Ubuntu system anyway) is:

/usr/local/lib/python2.7/dist-packages/Electrum-1.9.5-py2.7.egg/electrum_plugins/

( though the version number could be different )