The drivers are probably not installed. You need to establish what the make and model of the cards are. From a console do modconf as root. If it says command not found install modconf. modconf will bring up a menu listing all the driver modules. Go down the list until you find network cards and look for the one that matches. Install it. Networking is controlled through /etc/network/interfaces and it should look something like this;
auto lo
iface lo inet loopback
# This entry was created during the Debian installation
# (network, broadcast and gateway are optional)
auto eth1
iface eth1 inet static
address 10.0.0.10
netmask 255.0.0.0
gateway 10.0.0.2
auto eth0
iface eth0 inet static
address 192.168.0.2
netmask 255.255.255.0
broadcast 192.168.0.255
the NIC attached to the DSL needs to be on the same subnet and the one attached to the lan needs to be on the same subnet as the lan users. In the case above the modem/router uses the 10.0.0.X range and the lan uses 192.168.0.X range. The gateway for boxes behind this will be its IP address i.e. 192.168.0.2. They must be on different subnets. You initiate networking by bringing up a console and issuing the command /etc/init.d/networking start as root to restart or stop use restart or stop instead of start. You can check the networking with the command ifconfig -a.
If you want some more reading then have a look at the documentation at
http://www.debian-administration.org/index.cgi
If you need further info just ask.