pppd (point-to-point protocol daemon) comes with most Linux distributions. PPPD manages PPP session establishment and session termination.
You must also have in-kernel PPPoE support, which is included in all modern distributions by default.
Choose the DSL tab and press "Add"
Fill in the connection name (usually Cactus)
Leave the Service field blank.
Enter your username and password (provided by Cactus - phone 208 883 5500 for them)
You'll probably want to check "Connect automatically", "Show password", and "Available to all users".
Choose the IPv4 Settings tab and make sure the Method is "Automatic (PPPoE)