More fun with VirtualBox and OS X Mavericks. The message is quite clear, kernel extension not loaded.
Luckily, Jason Arneil worked through the problem and his solution worked for me: http://jarneil.wordpress.com/2013/10/24/vritualbox-4-30-on-os-x-10-9-mavericks/
Basically some leftover stuff from some VirtualBox 4.2.x installations had to be removed.
YMMV, good luck!