Hi,
no matter what version of Virtualbox I choose, 3.2 from debian unstable or 3.6 from the virtualbox side, the installer of the according extensionpack, needed for usb2 support, asks for the root password with a kdesudo requester. Givin it there fails. Can't remember whether this happend on my last install, too, but I guess not.
However, my workaround for now:
Opening root-dolphin via suxterm and opening the .vbox-extpack file there with root rights. No requester, pack installs.
greets
ayla