Guys, I had your problem, and I found a way around it. The problem revolves around using the web browser Safari. after the 10.6.8 update for Safari (to support OSX lion), the Java applet broke and therefore after Safari couldn't run anything big involving Java plug-ins (i.e. minecraft online).
I got around this issue buy downloading an
older version of the web browser Firefox, which can be found here:
http://www.mozilla.org/en-US/firefox/all-older.html
That above is Firefox 3.6, which, unlike Firefox 4, still supports Java plug-ins with OSX Lion.
The older firefox still works with OSX Lion and the Java applet, and fixed any problems that I had getting on Zombie Survival. Hope this helps!
Note: while loading a multiplayer server on Firefox, don't scroll or click anything on the screen except the 'Accept' button on the Java plug-in notification pop-up.
You may have to do this up to 5 times until it will allow you to play. Only then may you normally click.