Login to your LoginRadius account at www.loginradius.com and navigate to Deployment -> API Credentials
Here is a quick snapshot of the section on your LoginRadius user account where you can go to get the API key and Secret.
As of July 2014, after your free trial, only paid plans will be assigned an API secret. API secrets are only utilized in the retrieving of Social Data with the login interface which is no longer available under the free plans. You can view details on our plans on our pricing page