AquaMail has a preset for @att.net addresses, taken from stock Android Email (since I'm a looong way from AT&T's service area and don't have an account there):
Incoming:
- IMAP
- android.imap.mail.yahoo.com
- SSL (strict)
- port 993
Outgoing:
- SMTP
- android.smtp.mail.yahoo.com
- SSL (strict)
- port 465
These will be used automatically, but you can try pressing Manual on the first account setup screen, and removing the "android." parts (so it's "imap.mail.yahoo.com" and "smtp.mail.yahoo.com").
Of course it would help to know what error, exactly, you're getting. My telepathic skills are not that great