Access the installer inside the guest

Go to the running machine's window menu and select Devices -> Install guest additions. (This will cause VirtualBox attach a cdrom image with the installer to the guest machine in the background.)

Then mount the CD in the guest machine:

root@testkraxn / # mount /dev/cdrom /media/cdrom
root@testkraxn / # cd /media/cdrom

Run the installer

And run the install script after installing some tools if you didn't do yet.
This might take some minutes.

root@testkraxn /media/cdrom # apt-get install make gcc build-essential linux-headers-`uname -r`
root@testkraxn /media/cdrom # ./