If you want to archive from an email client then you only have to select mailboxes. For IMAP you need more information.
You need user name and server name if you use an email client. See article How to get user name and server name out of Mail, Outlook, Postbox or Thunderbird for IMAP? .
Password in Keychain
You need to get the password out of the Keychain - again if you use an email client. See article How to get passwords out of the Keychain .
For iCloud you need special settings and an application specific password. First you need an application specific password. See article How to use application-specific passwords for iCloud. Then you can add the iCloud account to Mail Archiver. See article IMAP settings for iCloud .
For Gmail you also need special settings and an application (and device) specific password. You first need to activate 2-step verification. Then you need to create an application specific password. See article How to create application specific passwords for Gmail. Then you can add a Gmail account in Mail Archiver.