About java plugin install, there is ppa repository available, therefore, the above step 1 and 2 can be replaced. (please visit http://www.webupd8.org/2012/01/install-oracle-java-jdk-7-in-ubuntu-via.html )
Now December, 2013 when I run Firefox 26.0 with Oracle java 7 (Version 7 Update 45) on Linux Mint 15, I can join the Microsoft Livemeeting test site at http://go.microsoft.com/fwlink/?LinkId=90703
I don't have to change the Oracle Java 7 Plugin Control Panel => Advanced => Security => Mixed Code. It can be kept as default.
When you click Join Meeting at above MS test site. You see couple of warning message then just click "Run" or "Allow" button to go ahead.
After couple of clicks, at the top-bar, you'll see pop-up block message. After you click the warning bar and allow pop-up show, you have to re-enter the meeting on web page.
Then "Microsoft Office Live Meeting" web console window is open, but it looks it doesn't move forward, if you don't touch anything.
There is java plugin icon at the left of location bar on this web console window. If you click this icon, you will see "Block" or "Allow" message window to run MS Live Meeting Java. Just continue to click "Allow" or "Don't Block" make MS staff run on the window. Finally you can join the test meeting.
Once you've done this for the first time and let firefox remember you allow all these things.. You will be much easier to join next time.
I've also confirmed that Ubuntu 13.10 does work above. Also I found that Chromium can run MS Live Meeting Java as well. I had been struggling with Chromium Java and I gave up using Chromium while ago. And now it's working. I'm not sure when and where this fix has been done.
(Update on Feb 18, 2014) Recent update on Java to update 51 causes Live Meeting to stop saying "Security Permission errors". This time, we need to change the security lever pulling down to "Medium" instead of "High".
(Update as of June 30, 2014) Java update 60 has no change. Continue to put the security level as "Medium" in Java plugin conrol panel.
(update on July 20, 2014) Java updated 65, but no change in above.
(update on September 9, 2014) Java updated 67. No change necessary.