Installing VBoxAdditions on Linux

You need to install kernel headers in order to install VirtualBox Additions. To do this in Fedora, execute:
  su
  yum install kernel-devel-$(uname -r)

(sudo apt-get install linux-headers-$(uname -r) on Debian based systems)

The uname bit gets the correct version for your particular machine, otherwise you'd get the latest which may not be appropriate.

You may also need to install gcc and dkms to get the module to build:
  su
  sudo yum -y install dkms gcc

(sudo apt-get install dkms gcc on Debian based systems)

Thanks to: http://digitizor.com/2009/05/26/how-to-install-virtualbox-guest-additions-for-a-linux-guest/