Compile forcedeth as a module.
Then to load add to your /etc/modules
# echo forcedeth >> /etc/modules
What this instruction does is add the text forcedth to the bottom of your /etc/modules.
This will load forcedth every time you boot.
You can also load it manually using modprobe.
# modprobe forcedeth
Another option is to actually build into the kernel.
Then you must try and configure the eth device.
Once the module has loaded devfs will create an eth device, /dev/eth0, /dev/eth1 etc...
depending on how many lan cards you have.
Try using the mandrake GUI config tool.
I have not used Mandrake since 9.1 and to get nvnet to work I had to configure it manually.
This may not be the case with forcedeth but I am not sure.
Anyhow I you search this forum you willl find an explanation of how to configure manually for mandrake. I wrote it a year ago.