If you have problems creating new folders for your IMAP Gmail (or Google Apps) account, please be aware that Gmail has a limit on the number of characters for a Label, and in Postbox, folder names are converted into Labels on the server.
So a folder in Postbox such as:
INBOX / Clients / Active / ClientName / AgencyName
gets translated into Gmail as a Label
and if that Label string is too large, Gmail (or Google Apps) will reject it.