For the record, a valid epub (zip) contains at its root level an uncompressed mimetype file and a META-INF folder that contains a container.xml file. This container.xml file tells you where to find the opf file (in Sigil this is always called the content.opf).
Here is a short example container.xml file:
Code:
<?xml version="1.0"?>
<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
<rootfiles>
<rootfile full-path="OEBPS/content.opf"
media-type="application/oebps-package+xml"/>
</rootfiles>
</container>