After installing Postbox, if you are unable to see or find your IMAP mail folders, then please check the following:
1) Ensure that you are subscribed to your folders. Please see: Subscribing to IMAP Folders.
2) Ensure that your folders are accessible to IMAP clients. Account specific information can be found below:
3) Verify that your IMAP prefix settings are correct. From Preferences or Options, select Accounts > your desired account > Advanced... button.
For Gmail and Google Apps accounts, make sure that "IMAP server directory" has an empty value, then restart Postbox.
4) If you are unable to view deeply nested folders, please see the Help Article.