Are you importing the right zip file? Sometime what happens is ,the extension which is downloaded from Marketplace contains zip extension inside the main downloaded zip file .So just verify that the zip which you are trying to import contains a metadata file immediately inside that .
Thanks for the answer.
Well, if I'm trying to import such zip (inner or external and with metadata-file in each case) it writes "Extension must have a metadata.xml file", although these folders actually contain metadata-file. So maybe I'm trying to import wrong zip (or wrong kind of archive?) anyway.
For example, there is a zip-file with extension like that. Into which folder should I paste a metadata-file for further importing as archive? Could you suggest, please?