Click the gear icon in the upper-right and select Gmail settings at the top of any Gmail page.
Click Forwarding and POP/IMAP
Select Enable IMAP.
Configure your IMAP client and click Save Changes.
When you first launch Postbox, it should take you to the New Account Setup page. If it does not, or you are adding an additional account, click on File > New >Mail Account
Provide your Nameand Email address, then click Manual Setup