I have installed a fresh copy of Linux Mint 13 in Virtualbox to experiment with and I needed to install the guest additions to enable full desktop resolutions and many other features. I downloaded the guest additions ISO from here: http://download.virtualbox.org/virtualbox/4.2.0/ and then added it to Virtualbox as a CDROM device. Once in Linux Mint I navigated to the Guest Additions folder and installed the kernel modules and other required drivers this way.
[email protected] /media/VBOXADDITIONS_4.2.0_80737 $ sudo ./VBoxLinuxAdditions.run [sudo] password for flynn: Verifying archive integrity... All good. Uncompressing VirtualBox 4.2.0 Guest Additions for Linux.......... VirtualBox Guest Additions installer You appear to have a version of the VBoxGuestAdditions software on your system which was installed from a different source or using a different type of installer. If you installed it from a package from your Linux distribution or if it is a default part of the system then we strongly recommend that you cancel this installation and remove it properly before installing this version. If this is simply an older or a damaged installation you may safely proceed. Do you wish to continue anyway? [yes or no] yes Removing existing VirtualBox DKMS kernel modules ...done. Removing existing VirtualBox non-DKMS kernel modules ...done. 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. You should restart your guest to make sure the new modules are actually used Installing the Window System drivers Installing X.Org Server 1.11 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.
After this I just restarted Linux Mint and I was able to use full desktop resolutions in a Guest operating system. Always download the Guest Additions that match your Virtualbox version.