Do you have a separate router? or are you trying to connect slackware to ADSL directly?
For the former, /etc/rc.d/rc.inet1.conf is the main configuration file for ethernet. If you'd like to get it up manually, do something like this:
ifconfig eth0 192.168.1.100 netmask 255.255.255.0 up
route add default gw 192.168.1.1
assume 192.168.1.0/24 is you network.
For the latter, you can probably use pppoe-conf to connect directly. Enter your username and password when it asks, and use pppoe-start.