Make sure that the API is enabled, otherwise on the main dashboard of the API manager select ENABLE API.
First, you will need to select the correct project. The project name in our case is "API Project", we take a note of it's id "api-project-1023838283680", this is the project id we will provide on mist.io.
Next, click on API Manager on the left sidebar and then Credentials.
Select Credentials, then Create credentials, and choose Service account key'.
Select Compute Engine Default Service Account (or give it another name if you want to distinguish the key).
The key is now created! Save the file.
Now use the mist.io add cloud wizard to add the Project ID and JSON key file.
Mist.io will automatically pull in all your VMs so you can see them alongside your other clouds.