If you install ubuntu as a guest on virtual box, you need to do a couple of things to install the virtualbox additions:

1. Mount your ubuntu cd/iso from the virtualbox GUI.
2. Install GNU Compiler and header files (use the command uname -r and substitute it with KERNELVERSION in the command below)

   sudo apt-get install build-essential linux-headers-KERNELVERSION

3. Now mount the VirtualBoxAdditions cd/iso from the virtualbox GUI.
4. Run the VirtuaBox installer:

   sudo sh /media/cdrom/VBoxLinuxAdditions.run all

5. Restart the virtual machine and you are all done!