Knowledge Is Everything

Boot the guest OS.Here it is Ubuntu 11.04.For installing guest additions mount the VBOXADDITIONS CDROM by clicking Devices -> Install Guest Additions as given in picture below.

Then you can see a cdrom mounted on your ubuntu desktop.Now open terminal and type

cd /cdrom
cd /media/VBOXADDITIONS_4.0.8_71778/

then if you have 32-bit system run the command as below

sudo sh VBoxLinuxAdditions.run

elseif you have a 64-bit system

sudo sh VBoxLinuxAdditions-amd64.run

or if you dont want to go like this,simply type

./autorun.sh

This command should handle the architecture as well as the privileges for you.

The installation screenshot is as given below.After that restart the guest OS(ie, here Ubuntu 11.04)