sudo apt-get install dkms
Insert Guest Additions CD image and install it
sudo usermod -a -G vboxsf <username>
/usr/lib/nux/unity_support_test -p
sudo apt install build-essential module-assistant dkms
sudo m-a prepare
Insert Guest Additions CD image and install it
Edit the file /etc/modules
by adding the line
vboxvideo
at the end of the file
sudo usermod -a -G vboxsf <username>