According to the guide, once the setup is complete you will need to:
1: In a new browser window, go to http://<instance-external-ip-address>:8088/login.htm
2: Sign in to Wowza Streaming Engine Manager. The default username is wowza and the default password is the Google Compute Engine instance ID (for example, 248889107416955973). To get your instance ID, use the SSH from the Browser feature in the console to connect to your instance, and then enter the following command line:
3: Follow the playback instructions to play the installed sample.mp4 video file using the test players in Streaming Engine Manager.
curl http://metadata/computeMetadata/v1/instance/id -H "X-Google-Metadata-Request: True"
I hope this answers your question.