For SBC Yahoo dialup, you can use either your SBC account information (such as username@sbcglobal.net) or the associated yahoo account. For my SBC Yahoo, I use the following information:
Account Description: Yahoo
Full Name: <Your Name>
Email Address: <YourEmail@yahoo.com> or <YourEmail@sbcglobal.net>
User Name: <your login name>
Incoming Mail Server: pop.sbcglobal.yahoo.com
SSL: off
Outgoing Mail Server: smtp.sbcglobal.yahoo.com
SSL: off
Alternately, SBC Yahoo offers an auto-setup wizard you can download and run from your desktop (for Mail or other apps).
Here is a link to download it:
http://help.sbcglobal.net/article.php?item=3990&fapplication=macosxmail&fos=macxall&action=Refresh
You will need StuffIt Expander to unpack it, depending on the age of your Mac you are likely have that already.
Cheers.