source: http://joergweis.wordpress.com/2009/03/07/install-virtualbox-guest-additions-on-ubuntu-server-810

run this: apt-get install build-essential linux-headers-`uname -r`
then install the guest additions package

another source: http://ipggi.wordpress.com/2010/03/11/virtualbox-shared-folders-with-ubuntu-server-guest
also describes how to mount virtualbox shared folders in ubuntu server
“mount -t vboxsf vboxsf vbox_shared_folder /media/vbox”