1. Mount the Guest OS Additions image file (ISO).
2. Open up a terminal, go to /media, and see if the image has been mounted successfully.
jayla@jayla-VirtualBox:/media/VBOXADDITIONS_4.3.10_93012$
3. Run the relevant script
sudo sh ./VBoxLinuxAdditions.run
4. Guest OS Additions should be installed in folder /opt/VBoxGuestAdditions-4.3.10
For more details, go to the official VirtualBox site
https://www.virtualbox.org/manual/ch04.html