$ cd /run/media/user/VBOXADDITIONS_4.3.20_96996/
$ sudo ./VBoxLinuxAdditions.run
You should see the following:
Verifying archive integrity... All good.
Uncompressing VirtualBox 4.3.20 Guest Additions for Linux............
VirtualBox Guest Additions installer
Copying additional installer modules ...
Installing additional modules ...
Removing existing VirtualBox DKMS kernel modules [ OK ]
Removing existing VirtualBox non-DKMS kernel modules [ OK ]
Building the VirtualBox Guest Additions kernel modules [ OK ]
Doing non-kernel setup of the Guest Additions [ OK ]
You should restart your guest to make sure the new modules are actually used
Installing the Window System drivers
Installing X.Org Server 1.16 modules [ OK ]
Setting up the Window System to use the Guest Additions [ OK ]
You may need to restart the hal service and the Window System (or just restart
the guest system) to enable the Guest Additions.
Installing graphics libraries and desktop services component[ OK ]