Incredimail is constantly slated in the technical press and widely reviled elsewhere. It's slows some computers and drives recipients of mail sent through it up the wall because of the way that it assumes that everyone uses HTML mail (when many people don't). My advice is AVOID, AVOID, AVOID!
However, if you're determined to stick with it (WHY???), go to Tools > Email Accounts and REMOVE any account that you've already got there.
Close Incredimail, restart your computer and open Incredimail again.
Go to Tools > Email Accounts again and select 'Add'.
In the wizard that opens, select 'Other'.
Complete your email address and password and click 'Next'.
Set the incoming mail server type to 'IMAP'.
Complete the incoming mail server field thus: imap.tools.sky.com
Complete the outgoing mail server field thus: smtp.tools.sky.com
Try accessing your mail. If you still can't get in, go back to Tools > Email accounts. Click on the name of your account. Click on Properties. Check that any references to 'Security type' show 'SSL', that the incoming server port is set to '993' and the outgoing one shows '465'.
[NB: The above will probably work but Sky Yahoo Mail doesn't officially support Incredimail, so I can't guarantee it!]