Google does not allow openid authentication through google apps premier accounts by default. (Standard edition google apps accounts work fine).
To change this, you need admin access to the google apps account:
For domain administrators, Go to Manage this domain > Advanced tools > Federated Login using OpenID
Check Allow users to sign in to third party websites using OpenID.
Please note that installing the Google apps marketplace app will insert a horizontal Freshdesk bar into every user's email account in the domain. This is out of our control, and is a part of Google's default behavior for marketplace apps.