Telnet/SSH and the Command Line

http://www.dd-wrt.com/wiki/index.php/Telnet/SSH_and_the_Command_Line

Open your favorite Telnet client (On Windows, you can use Start > Run > telnet )
connect to e.g. 192.168.1.1
When asked for the username, enter root (even if you changed username in web interface)
When asked for the password, enter your router's password