Hi im currently very new to linux and i am having a problem updating my madwifi driver to madwifi-0.9.4 on Backtrack 3. (USB Version)

i have followed all the steps on getting rid of the old drivers but while trying to run make && make install its displaying the following error.

Code:
/bin/sh: line 0: cd: /lib/modules/2.6.21.5/build: No such file or directory
Makefile.inc:66: *** /lib/modules/2.6.21.5/build is missing, please set KERNELPATH.  Stop.
i understand i have to add in a few more lines of instructions to make this work but i just don't know what.