Current questions and answers regarding the map editor.

I replaced .zip with .h5u in the name, but what should I have done instead?

If it's just in the name, it's useless. The file will still remain a "zip" type, even though it will be called ***.h5u

I usually take an existing mod with the .h5u extension, copy it, add what it should contain, and then delete the old content.

In general, it's best to change the extension using Notepad.
Open it with Notepad, and save it as a non-text file, specifying the extension yourself.