Try upgrading to the latest Java release, or change your Java security settings (via the Java control panel) to allow applets to run with older Java releases.
I have the same problem and I have never touched my settings. Maybe a third party program mucked it up. Who knows... Could someone help me?
go to systemcontrol then programms, java, security, and then configurate your whitelist there
Sorry forgot to say that I already figured it out. But in case other people want to know here is a good tutorial.
Do a search for "java control panel" without the quotations.
Click on the java control panel icon.
Enable java in browser.
No need to change security settings.
Restart browser.
Done!
If not working put security settings to lowest and add www.greenfoot.* to your whitelist/greylist.