Assuming you have Yahoo Mail Plus, you use:
pop.mail.yahoo.com
and
smtp.mail.yahoo.com
for your incoming and outgoing mail The outgoing port must be 587
Everything else is about as you'd expect when you add a pop e-mail account.
Again, this ONLY works if you're paying for the premium Yahoo mail service.
If you go to their site, they have specific instructions for Outlook,
Outlook Express, Eudora and a generalized method setup.,