OpenSuse with virtualbox

Hi and first sorry for bad english.

I’m giving a try to the last version of OpenSuse. I have performed an installation via Virtualbox on my kubuntu (lucid). The installation was ok (I chose KDE), but after the splash screen, I just face a black screen. ALT+F2 works but I can’t launch any KDE application. I succeed in lauching firefox but that’s all. I tried launching plasma-desktop but it does not have any effect. Any idea of the origin of the problem ?