As far as I know the following works on mountain lion, mavericks and yosemite - 10.8, 10.9 and 10.10.
First close all your browsers.
go to finder Go > Go to folder ~/library
in internet plug ins folder delete WebEx64 plug in
in Application Support folder delete the webex folder
go to http://www.webex.com/test-meeting.html
enter name and email you should then get a prompt to install the plugin. It should then start up a test meeting.