you are making the link in /usr/src
ln -s linux-2.4.18 linux
that will give you a symbolic link from /usr/src/linux to /usr/src/linux-2.4.18
which may fix the error in your makefile.
You can also edit the makefile by hand, changing the portion that directs it to /usr/src/linux to /usr/src/linux-2.4.18 although that seems a bit overkill for this situation.
Which Linksys card do you have?? I ask because I've yet to have to compile drivers for a wireless PCMCIA card since the 2.4 kernels........
maybe I'm just getting lucky though
best of luck,