apt-get install linux-source-2.6.18
But this line and similar variations give back nothing.
That's a command for installing from a repository. Unless you've named your CDs as sources for installation, apt-get should have given you some kind of error message.
The previous post assumes you have already installed the source. If not, you could just read it from the CD.
You could also get the source directly here: http://kernel.org/