I wanted to install Cisco VPN client on BackTrack3. You can get the Cisco VPN client source using the following command:
wget ftp://ftp.cs.cornell.edu/pub/rvr/upload/vpnclient-linux-4.8.00.0490-k9.tar.gz
tar zxvf vpnclient-linux-4.8.00.0490-k9.tar.gz
cd vpnclient/
wget http://tuxx-home.at/projects/cisco-vpnclient/vpnclient-linux-2.6.22.diff
patch < vpnclient-linux-2.6.22.diff
./vpn_install

I got this information from the following blog.
I ran into an error whereby the kernel sources were not found for the VPN client to install. I then got the BackTrack3 kernel sources.
cd /lib/
wget http://www.offensive-security.com/kernel.lzm
mkdir test
lzm2dir kernel.lzm test

Now go into the vpnclient directory and execute the following:
./vpn_install

Accept the defaults (except in my case I selected “No” on automatically start VPN client). When it asks for the sources point it to:
/lib/test/usr/src/linux-2.6.21.5

Then the VPN Client should compile without any issues. Then you just need to place your Cisco VPN client Profile (.pcf) in the /etc/opt/cisco-vpnclient/Profiles directory. You will need to first start the VPN client service first using:

/etc/init.d/vpnclient_init start

Once the service is started just connect using:

vpnclient connect mypcffile user test password <whatever>

Please note that the full name of the Profile file in the above case is mypcffile.pcf but I’ve deliberately excluded the .pcf extension.
This should work.