Make sure you install Kernel Headers Package.
http://www.cyberciti.biz/faq/howto-install-kernel-headers-package
Make sure you have updated version
Type the following command
$ sudo apt-get update
OR as root user enter:
# apt-get update
Search for kernel version (optional)
Type the following command:
$ apt-cache search linux-headers-$(uname -r)
Install linux-header package under Debina or Ubuntu Linux
Type the following command:
$ sudo apt-get install linux-headers-$(uname -r)
OR as root user:
# apt-get install linux-headers-$(uname -r)
EDIT:
I just happen to need to install it also on my own server and got the same error when trying to use the virtualbox in debian repos. I went ahead and added VirtualBox repo and installed from there. This is better because they are more up-to-date.
As root
$ nano /etc/apt/sources.list
Add line below for squeeze
deb http://download.virtualbox.org/virtualbox/debian squeeze contrib non-free
Then add the The Oracle public key for apt-secure
$ wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -
then run update and to install latest 4.2
apt-get update
apt-get install virtualbox-4.2
Install the dkms package to ensure that the VirtualBox host kernel modules are up to date
apt-get install dkms
Should be good to go. Got these instructions from VirtualBox's website for Linux which seems to be down at the moment but the link is https://www.virtualbox.org/wiki/Linux_Downloads