linux kernel source code

I need help locating kernel source code directory,
debian.org said it is located at /usr/src/linux, but it is not there. The reason I'm asking
is I'm trying to install something "client for wireless accsess to net" just a progam to access the web through my school, It is asking me for linux kernel source code directory so it can build some kernel module.Where is it located, or are there any alternatives?

Did you actually install the kernel sources package in the first place?

It's linux-2.* in /usr/src directory, not just "linux". That "linux" symlink isn't used anymore. And yes, you need to install the kernel-devel package.

No, I didnt install anything, I thought it came with package when I installed the system, how do I check which kernel I'm using, and how/where to install it. I have synaptic package manager. And what is the purpuse of this source code?