Mailbird has full import from Thunderbird, Outlook and Windows Live Mail. Import instructions are given in this article. However, some of the users may face the issue when Mailbird cannot find the existing profile for import.
Mailbird would look for your profile for import in this folder c:\Users\[username]\AppData\Local\[Microsoft or Thunderbird]\
But sometimes this location can be changed, and the profile moved.
So the first thing to do is to find if you've moved your Outlook profile folder or not. If you did, please move it back to the default location and try import again.
If you still face the error, please contact us at firstname.lastname@example.org