Email archives must be opened from within Outlook (not by double-clicking, for instance). The procedure is as follows:
The opened file will appear in the left-hand pane of Outlook, alongside the connected email accounts.
The archive file you have opened can be used like an email account. You can open items in it just the same way as in email accounts. You can create new folders in it by right-clicking on it, or you can drag folders from other accounts and drop them into it. You can also drag individual emails or groups of emails into folders in the archive file.
Note that when dragging emails or folders from one place to another in Outlook, the default behaviour is to move them, not to copy them.
It is also possible to store other Outlook items such as contacts in the file.
When you have finished working with an archive file, you can disconnect it from Outlook by right-clicking on it and selecting Close.