If you are setting up a new iCloud account, please follow steps 1-5.
If a previous iCloud account is no longer working, follow steps 2-5.
1) Set up your iCloud account as if it were a me.com address within the Postbox Account Wizard, however, do not enter a password.
2) After creating the account, go to Preferences | Accounts | <account> and change the IMAP server from mail.me.com to imap.mail.me.com
3) RESTART Postbox! This is important!
4) You should be able to connect to your free iCloud account. Your password is the same as your AppleID password.
5*) IMPORTANT: Some users have experienced success with changing their password at https://appleid.apple.com, then within Postbox. If steps 1-4 do not work for you, then please change your password. Also be sure to include a number and a capital letter in your new password.
There are many reports on the web of mail credentials not working in Postbox, in addition to Sparrow, Thunderbird, etc. For examples, please see these Apple Forum Posts: