Can not install Guest Additions in VirtualBox for CentOS 6 guest

You need to install guest editions.

"The VirtualBox Guest Additions for all supported guest operating systems are provided as a single CD-ROM image file which is called VBoxGuestAdditions.iso. This image file is located in the installation directory of VirtualBox."

create mount directory

sudo mkdir -p /media/VirtualBoxGuestAdditions

mount guest additions iso

sudo mount -t iso9660 -o loop /installation/directory/of/VirtualBox/VBoxGuestAdditions.iso /media/VirtualBoxGuestAdditions/

Install guest additions

sudo /media/VirtualBoxGuestAdditions/VBoxLinuxAdditions.run

Then mount.vboxsf file should be in sbin and you can mount with

sudo mount -t vboxsf shared ~/host 

See https://www.virtualbox.org/manual/ch04.html

After following the steps described by the given answer and experiencing a kernel header issue i came across this link which has a command that does simplify things.

/etc/init.d/vboxadd  setup

Hope it helps future explorers on this question. :)

I got the same error.When I tried to install VBoxGuestAdditions (Devices -> Insert Guest Additions CD Image) on Ubuntu 16.04 x86, it throws error message "Unable to insert the virtual optical disk ..."

SOLUTION:1. Open a terminal and enter: sudo apt-get install virtualbox-guest-utils

2. Reboot and enjoy.