This is how i did it earlier today. It will only work with Wifi off. Once you get it working and later were to turn Wifi on, and then open the yahoo email it will say, "invalid username or password." Then you get to set it all up again. it sucks but if you can do without Wifi until a better solution comes around you can save some money and not buy the PLus Yahoo.
Make sure Wifi is off. Reboot the phone.
go to mail icon
add account
enter email address in email address filed (name@yahoo.com)
enter password at password field
select MANUAL SETUP not next
select imap
username=
name@yahoo.com
password = yourpassword
imap server= imap.mail.yahoo.com
port=143
security=none
optional = blank
select next
you will then be prompted to sign in do so.
will then be taken to the smptp settings which only prompts (going form memory) you for address which is
smtp.mobile.mail.yahoo.com. leave the port as is. then select next. sign in. name your account and that's it.