when you do: "modprobe forcedeth" (without quotes) what do you get?
If it says nothing, check with "lsmod" (without quotes) to verify it has been loaded. If it gives an error, it means you probably didn't build your kernel with forcedeth support. I guess your using Gentoo, since that's what it says below your name. So if you have allready compiled your kernel using genkernel this might be a simple way:
genkernel --menuconfig all
then: Device Drivers -> Network Device Support -> Ethernet (10 or 100mbit) -> (M) Nforce Ethernet Support.
Then quit save and it'll do the rest for you automaticly. May take some time.
If you didn't do it with genkernel your first time, just go:
cd /usr/src/linux && make menuconfig
And do the same as above, and then remake the kernel:
make && make modules_install
cp arch/i386/boot/bzImage /boot/kernel-2.6.19-gentoo-r5
rename to whatever suits your needs.
Then add forcedeth to autoload:
echo "forcedeth" >> /etc/modules.autoload.d/kernel-2.6