Fix "Unable to complete activation. Device not registered"
By If the UDID your device is not activated as registered device in iOS Provisioning Portal, you will most likely getting this error on iTunes after restore to iOS beta.
We’re unable to complete your activation.
This device is not registered as part of the iPhone Developer Program.
If you are a member of the Program, please register your device in the iPhone
Developer Program Portal.
In order to install iOS beta version (such as iOS 4 beta or iOS 5 beta), you will need to be registered as iOS developer and add device ID (UDID) to provisioning portal.
Otherwise, if you don’t want to pay $99 only to let you trying new features iOS beta version, you can ask existing developer (family, friends, someone forum, etc) to add your device to provisioning portal for free. If you don’t know anyone that will let you add UDID for free, you can spent a few bucks to activate UDID using online services which offering UDID activation service.
To see the best and trusted UDID activation services, click here!
It’s a good deal to pay a few bucks instead paying $99 to enroll yourself as registered iOS developer.
Activation 
