sudo sh ./VBoxLinuxAdditions-x86.run x11
You might see similar to the following error
Warning: unknown version of the X Window System installed. Not installing
X Window System drivers.
One possible explanation to this is that the 2.1.4 vbox video drivers do not yet support Xorg version which ships with Alpha 6.
Work around for this problem
Mount the VirtualBox Guest Additions CD. You can do this from Devices > Install Guest Additions
Applications > Accessories > Terminal and enter the following command to enter in to you cdrom
Extract the the installer
Note:- Select your hardware architecture file to extract i.e x86/amd64 and vbox is sample directory you can choose where ever you want to extract.
Now you need to edit the install.sh file (Inthis case i am in vbox directory)
sudo gedit install.sh
In Line 415 change
1.5.99.* | 1.6 )
1.5.99.* | 1.6.0 )
Save and exit the file
Note:- This is needed, because the version that is returned by X -version is 1.6.0, not 1.6.
Now run your installer using the following command
After completing installation Reboot your system.
That's it enjoy your virtualbox
Source from Virtualbox forum