818 - No JDK

I feel a bit stupid asking this, but anyway...

Having loaded 818, it now complains that there is no JDK. I've gone to the "Default Project Properties" dialog to select one but no matter what directory I select I get the error dialog "The directory selected is not a valid JDK home.".

Could someone enlighten me on what constitutes a valid JDK home.

Thanks,
Vince.