As the error says, the mimetype file has to be the first
file in the zip, and must be stored without compression.
What I do (on linux) is create the zip/epub file in two steps. First create the zip only with mimetype, without compression:
zip -0 file.epub mimetype
Then add the rest of the files with maximum compression:
zip -9 -r file.epub META-INF OEBPS
You can probably do the same with Winzip or similar.