The default installation of Ubuntu for your desktop doesn't install dkms package. Following the (*) guide you will have to install it.
Additionally you have to install your kernel source header files that
apt-get install aptitude aptitude install linux-headers-3.5.0-26-generic
After this the driver instillation goes well. All what you need to do is now to restart your VM.
root@rado-VirtualBox:/media/rado/VBOXADDITIONS_4.2.10_84104# sh ./VBoxLinuxAdditions.run uninstall root@rado-VirtualBox:/media/rado/VBOXADDITIONS_4.2.10_84104# sh ./VBoxLinuxAdditions.run Verifying archive integrity... All good. Uncompressing VirtualBox 4.2.10 Guest Additions for Linux.......... VirtualBox Guest Additions installer Removing existing VirtualBox DKMS kernel modules ...done. Removing existing VirtualBox non-DKMS kernel modules ...done. Building the VirtualBox Guest Additions kernel modules ...done. Doing non-kernel setup of the Guest Additions ...done. Starting the VirtualBox Guest Additions ...done. Installing the Window System drivers Installing X.Org Server 1.13 modules ...done. Setting up the Window System to use the Guest Additions ...done. You may need to restart the hal service and the Window System (or just restart the guest system) to enable the Guest Additions. Installing graphics libraries and desktop services components ...done.