I took for granted that you had allready compilled the kernel.
please type this and post the result.
All this does is print to stdout(terminal) kernel version actually running.
If it prints something like this...
Then I think you should be good to go allready.
Just change directory to /lib/modules/2.6.3-1mdk/kernel/drivers/net.
# cd /lib/modules/2.6.3-1mdk/kernel/drivers/net
and unpack the forcedeth.ko.gz.
# gunzip forcedeth.ko.gz
Now you will have the module installed.
You can now try to load it.
# modprobe forcedeth
Now you can follow my previous post.