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:/
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.