1) Installing Vbox guest addition using VBox option, where we choose from the virtualbox menu "Devices" and then Install guest additions does not work directly. So run terminal and log in as the root with choosen password. Then:
apt-get update apt-get install -y linux-headers-$(uname -r)
2) Mount the CDROM media with the additions (for example through VBox Devices menu).
3) Now copy run script from the cdrom (named VBOXADDITIONS_4_2.18_88780 in my case) to somewhere where we are able to run it
!list cdrom with inserted VBOX Additions ls /media cdrom cdrom0 VBOXADDITIONS_4_2.18_88780 cp /media/VBOXADDITIONS_4_2.18_88780/VBoxLinuxAdditions.run /root/ cd /root ./VBoxLinuxAdditions.run
4) Now we have to reboot Mint linux to complete guest additions installation complete.