In WinRar, when you select your files to combine, then click the Add button, you get a window that should look close to this:
Change the Archive format to ZIP.
Change the Compression method to Best.
Then click OK.
This will create a file with an extension of .ZIP.
Then, like Fern mentioned, right-click on this file.
Select Rename.
Change the .ZIP to .ztd
Then copy to your game folder and test it out :D