The manufacturer field was added in later version of the program so you will have to specify a version 3 request to get it.
You can call "QBPOSXMLVersionsForSession" from your request object to ensure that the end user has a compatible version of POS for your request.
This shows what fields are available to what versions: https://member.developer.intuit.com/qbSDK-current/Common/newOSR/index.html