Adding Trusted Publishers for Office 2013 via OCT or Group Policy

Hi,

I'm getting ready to deploy office 2013 to our environment with a GPO configured with the following setting enabled:

  - Require Application Add-ins to be signed by trusted Publishers

With this GP setting enabled, when I open a New, blank doc in Word I get the following notification:

 - Security warning: Some active content has been disabled. Click here for more details

and

 - Warning: The digital signature is valid (signed) but the signature is from a publisher you have chosen not yet to trust (it's actually Microsoft who are the publisher which is not yet trusted)

How can I add Microsoft and publishers into Trusted Publishers via OCT or Group Policy? I haven't been able to find anything clear on this after a extensive googling and self-investigation.

Thanks.