I hope I was thorough enough and clear enough with the instructions, I'm sure you'll let me know if I wasn't. It's not the best of instructions, but should do the job. As I've said before, I'm not very good at this sort of thing.
So, if you have any problems with the installation process, let me know.
Here the link to the MirrorBones plugin. http://www.sgross.com/plugins/
[EDIT] Updated relationships and added installation notes. To fix a model that has the previous version of the rig (update 9), just drag and drop the relationship folder from the installer to your models name in the PWS. You will need to reexport your model from an action, following the instructions.