If the VitualBox Guest Additions CD from Oracle Virtual Machine doesn’t mount correctly when you click on the Menu “Device->Install Guest Additions”, simply open up your root command prompt on Linux and enter:
mount /dev/sr0 /media/cdrom
Some more recent VirtualBox editions have renamed the file to run into:
./VBoxLinuxAdditions.run from ./VirtualBoxLinuxAdditions.run
And hopefully it should start the installation and if you linux vm is correctly installed and configured, it should run all nicely.
If not, try to upgrade your linux… (this depends on your distro)
For debian based linux versions it is:
apt-get update && apt-get dist-upgrade