I also have SBC Yahoo DSL and a 5100 speedstream modem, but with a hard drive install of slackware 10.0, 2.4.26
Assuming all of your connections are right, you should just be assigned (at least this is how it worked for me) an ip address by the isp. The only extra step that I had to do was run dhcpcd as root.
so i guess this would be my suggestion:
turn off the modem
open a console
run "dhcpcd" as root
turn on the modem
after a while (blinking lights are solid green), try ifconfig and see if you have a public ip address
test connectivity, ie "ping www.google.com"
not the best solution, but it worked for me!