These are the directions needed to configure your IMAP access in your Yahoo account:
For Incoming Server use imap.mail.yahoo.com
For Outgoing Server use smtp.mail.yahoo.com
For Incoming Port use 993 ( this requires SSL)
For Outgoing Port use465 (this requires SSL) or use port 587 (this requires SSL/TLS)
Yahoo Mail will use your full email address as the login name.