Actually, I should have asked you: how exactly did you try to open it? And also, why didn't you just put it in the resource packs folder in the com.mojang folder?

I can help you out with installing it manually.

The only reason I ask is because changing the file extension doesn't work for me; it tells me that it is not a valid zip archive.

Of course I can! I'll try to update it with a mc.pack file instead of a zip