The command you used for installing the kernel source is not right. It is supposed to be:
apt-get source linux-source-2.6.18
Check first the kernel version you are running with 'uname -r'. You should install the source of the same version you are running.
Be sure however that you have a line in your /etc/apt/sources.list which says:
deb-src
http://ftp.debian.org/debian testing main contrib non-free
(Substitute "testing" for "stable" or "unstable" as needed)
Then when the download is succesful do:
ln -s /usr/src/name_of_the_folder_containing_the_source_now linux
Refer to
this page to get all information I omitted or provided in error.
At the end you might want to build your own kernel to be sure that the source tree is really the source used for building the kernel. It is not scary as long as you double-check /boot/grub/menu.lst to see that your old kernel still is one of the boot options.
jlinkels