It's not /dev/eth0, eth0 is just an alias and can exist (and does exist) without a /dev entry. I have no /dev/eth0 entry on slack 10 and I'm on the internet right now
If you modprobe 3c59x then use
ifconfig eth0 192.168.x.x up
to set your ip address to 192.168.x.x (whatever you would use) you should be ok. Try pinging something. Type ifconfig by itself to see if it detects the card OK.