Webmail is available for all Mailstore accounts. If you do not have a Mailstore type account, you must first upgrade to Mailstore.
If you have a Mailstore account,you can log into webmail immediately at https://webmail.pobox.com
If, in addition to webmail, you also want to read your mail using your email application, please make sure your account is set up in your email program using IMAP instead of POP. IMAP synchronizes your mailboxes on the server, so the Inbox you see in webmail will always match the Inbox you see in your email application. (POP was designed to download mail to the application you used to read it. Although you can tell POP to "leave mail on the server", it was not designed to work that way, and tends to get confused when multiple applications try to access the same mailbox.)