I'm not really sure what this error means.

[Samurai]

But if I rename the zip file to .pk3, then I won't even have the option to unzip it. If I right-click it, it will ask if I want to zip it, not unzip it.

It's definately a pk3 file which CoD needs to know to be able to run it.

So if you don't need to do anything to it, just rename the file ext to .pk3 and CoD will do the rest.

As for opening it, you have the option to do this when you right click (Open With)

[tesco]

But if I rename the zip file to .pk3, then I won't even have the option to unzip it. If I right-click it, it will ask if I want to zip it, not unzip it.

go into winrar and click file>open then point to that file.