It's java based so your browser may not matter. Make sure you have the latest version of java
and your security settings aren't preventing it. If you're using a Mac you might want to try the oracle version of Java vs the default mac java.
OS X can be a Java compatibility nightmare.
I clicked the "Missing Plug-In" link and it took me to the Oracle Java page and I downloaded and installed it and it still wasn't working. I'm going to try it one more time this morning on the iMac with Firefox and I'll also try to search around it some more troubleshoot it. If not, I'll try the MBP.