This is the operations repo. The application repo can be found here. You will want to fork the application repo and use that with Atlas.
Make sure that you have created a new project in GCE with the name letschat-example-YOURATLASUSERNAME. You will also need to click credentials under API&Auth in the sidebar, click add credentials, select service account, check json, click create, and copy the file as account.json in both the packer and terraform directory.