We have taken iOS developer Enterprise account and we have received the MDM certificate which is in .pem format.We have download this mdm certificate from the portal https:/
My suggestion is try production APNS you can use this production APNS for live MDM product also.
If you are following this page, then take care of few things and see the question also.
Follow this page: http://www.softhinker.com/in-the-news/iosmdmvendorcsrsigning
In order to use the sandbox APNS service, tick 'Use Development APNS Server' in the Mobile Device Management section in the iPhone Config Util of the configuration profile you will create to enrol your device with your server.