Using a service account to push cards

Question

I'm trying to create a php page which will authenticate using a service account and will create a new timeline card for the application users.

(It's working using a client ID for web sites)

So I have created a service account, uploaded the key.p12 file to the server and I keep getting this error message:

Uncaught exception 'Google_Auth_Exception' with message 'Error refreshing the OAuth2 token, message: '{ "error" : "invalid_grant" }''

As I understood I also need to give access to the service user (the email address generated) in my app (I saw an example where this is how its done with Google Analytics) but where can I do it with the mirror service / my google glass application?

Basic question is: How do I authenticate with mirror using a service account.

Thank you!