Wireless instalation on Dell M20

I had etch 4.0 working perfectly but decided to install instead the new lenny 5.0 (not upgrade). Everything is fine but the wireless connection is not working. I remeber it was easy to do in etch but now I am confused.

If I am not wrong, first I need to install ieee80211. But this gives me the following problem:

dh045077:/home/carlos/ieee80211-1.2.18# make
Checking in /lib/modules/2.6.26-1-686 for ieee80211 components...
find: `/lib/modules/2.6.26-1-686/build/': No such file or directory
egrep: /lib/modules/2.6.26-1-686/build//.config: No such file or directory
egrep: /lib/modules/2.6.26-1-686/build//include/linux/autoconf.h: No such file or directory
find: `/lib/modules/2.6.26-1-686/build/': No such file or directory
make -C /lib/modules/2.6.26-1-686/build M=/home/carlos/ieee80211-1.2.18 modules
make: *** /lib/modules/2.6.26-1-686/build: No such file or directory. Stop.
make: *** [modules] Error 2
dh045077:/home/carlos/ieee80211-1.2.18#

The installation text file says that for the above problem:

The installation requires the compiled kernel sources or headers
against the matching kernel. These are typically found in:

/lib/modules/\`uname -r\`/build

If that directory does not exist, or is empty, you likely need to
install the kernel source packages for your distribution. Once you have
the kernel sources, you can make and install the ieee80211 subsystem
via:

% tar zxvf ieee80211-${VERSION}
% cd ieee80211-${VERSION}
% make

Where can I get those kernel source packages ? Is that not risky ?

Thanks in advance