Yahoo's POP3 support is open only to paid users.

POP3: Post Office Protocol v3, used to download e-mails.
IMAP: Internal Mail Access Protocol, a replacement for POP3 with more advanced mailbox management functionality.
HTTP: Web access...

If you do want to use POP for your Yahoo account, you may check out YahooPOPs, which is a freeware local POP to HTTP proxy for your Yahoo! account.