Note 3
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 or Ubuntu Server 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.