I have downloaded the Ork Framework demo game to try it out, but the Asset package has a .tar file extension and is not recognised when I try to import it into Unity. I understand I need to convert the package to a .unitypackage but cannot see how to do it
.tar files are archives, like .zip files. You need to extract the content of the archive and repack it into a unity package (or maybe not?).
https://wiki.haskell.org/How_to_unpack_a_tar_file_in_Windows
Thank you. I have extracted the files. How do I repack them into a Unity package?
Maybe you don’t need to. What does the content look like? How is it organised? Do you have an Assets folder containing scripts, prefabs and so on?
Yes, it has an Assets folder, Library, Project Settings and so on. Can I get these into Unity?
Absolutely! Launch Unity, select “open” and then find the main directory, the folder that contains the Assets and other folders. Once this done, you can either play in the editor itself or build the game.
AnneSchmidt you are a saviour. It works perfectly. Many thanks. Have a great weekend.
You’re welcome! Have a great week-end too! 
Sorry to keep going on, but now I have another problem. As above, I can now open the demo game and play it, but the tutorial resources that I need to go through the making of the game, are in a separate package with the usual Unity symbol -
and this .tar file is the one I can’t open in Unity as it is not a .unitypackage file