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 (sudo su). Then:
apt-get update apt-get upgrade apt-get intstall dkms
2) Mount the CDROM media with the additions (for example through VBox Devices menu).
3) Open terminal and copy run script from the cdrom (named VBOXADDITIONS_4_2.18_88780 in my case) to somewhere where we are able to run it
cp /path_to_your_cd_rom/VBOXADDITIONS_4_2.18_88780/VBoxLinuxAdditions.run /root/ cd /root ./VBoxLinuxAdditions.run
4) Now we have to reboot Lubuntu linux to complete guest additions installation complete.