I guess you are using ClickOnce to deploy your addin.
In this case you will have to sign the manifest by using a certificate.
Try to sign it by following the steps in this article: http://msdn.microsoft.com/en-us/library/bb772096.aspx
You can also refer to here for more information on signing and trusting VSTO solutions: http://msdn.microsoft.com/en-us/library/bb772086.aspx#signing