You need to set the key in the manifest file, based on your generated pem file.
There's instruction on getting the pem here: https://developer.chrome.com/extensions/packaging
And there's more detail on the key field here: https://developer.chrome.com/extensions/manifest/key
Make sure you save your pem file once you generate it! It can never be generated again, and you'll have to republish your app to get a new one.
