To test the SSO functionality of a particular environment, select the environment from the drop down list and enter the external_system_id of a user in the 'SAML SSO ID' box and click login.
To add an additional environment to the drop down, add a configuration section to the 'Saml.Configuration.ServiceProviders' area in the web.config (you will need to ensure the IssuerName coresponds with a valid SAML_PROVIDER in the environment you are adding.