I have setup Webstorm to recognise and compile my coffeescript files.
I have installed the AngularJS plugin and can successfully write my controllers etc in plain o
To fix the specific problem Unresolved variable or type angular you need to make sure the angular javascript libraries have been Enabled (see image below).
You do this via the WebStorm settings:
File -> Settings -> Javascript -> Libraries

Here is the relevant help section: Configuring JavaScript Libraries