Folder are available only via IMAP connection.
It sounds that you might be using a POP3 connection with your Comcast account.
If that's not the case, - you can read in the same FAQ item about how to enable folder syncing.
I'd highly recommend using IMAP instead of POP3, for many reasons.
You cannot change the type of the account in Aquamail, you will have to add that account with an IMAP connection and remove your POP3-based configuration.