The guest additions is mounted as a virtual CD visible on the launcher:
In case the system is set up to autorun a CD it should just open the following autorun dialogue (if not select the CD symbol, choose open, then choose Open Autorun Prompt )
After having entered your credentials for root access the guest additions will build as seen from the terminal output:
Press Return to close the terminal.
Guest Additions need a reboot of your guest OS to take effect.
In some systems (e.g. when running Xubuntu guests) we may not have an Autorun Prompt for the mounted CD. Then we can install the guest additions by opening a terminal on the mounted guest additions .iso to run sudo ./VBoxLinuxAdditions.run. In Lubuntu guests we additionally need to install the GNU C compiler (gcc ) and the make utility in the guest system to be able to compile the guest additions.
Running DKMS in the guest OS will
keep Guest Additions installed after a guest kernel update. It is not a prerequisite for Guest Additions but if used it should be installed before we install the Guest Additions.
Installing Guest Additions from a Terminal
In case Guest Additions are not installed properly we may not be able to boot to the GUI. In this case we need to boot holding the Shift key to access the Grub menu where we access a root shell as depicted in the following question (we do need to mount the filesystem read/write to proceed):
The Guest Additions .iso file needs to be installed on /dev/cdrom from the Virtual Box Manager (see above). We then may install Guest Additions by typing the following commands:
Remove Guest Additions
In case something went wrong with installing the guest additions, or the guest OS can not boot after installing we may have to remove the additions. This can be done by running an uninstallation script located in the guest /opt directory.
Replace x.x.xx with your current version of guest additions.