From the apple menu in the far upper left corner of the screen, select Control Panels then TCP/IP.
If you already have a configuration for TCP/IP using a dialup modem, go to File and Configurations. In that panel, create a new confirguation and name it "DSL."
In the settings panel for the new "DSL" configuration.
For dynamic IP:
Uncheck "use 802.3"
Select Automatic configuration and Ethernet.For static IP:
Select Manual configuration and Ethernet.
Uncheck "use 802.3"
Enter the configuration information provided (IP address, netmask, gateway, DNS server)
Note that this configuration does not affect the dialup TCP/IP configuration. The customer may run both simultaneously. The TCP/IP configuration also does not affect other protocols such as Appletalk. The workstation should still communicate properly with non-TCP/IP devices on the Ethernet network.
If the connection doesn't work immediately, you can test it. However, the Mac does not come with a ping utility. You'll have to use a dialup connection to download MacPing (shareware) to send a test ping to the gateway.