To use the cookies API, you must declare the "cookies" permission in your manifest, along with host permissions for any hosts whose cookies you want to access. For example:
{ "name": "My extension", ... "permissions": [ "cookies", "*://*.google.com" ], ... }
You can find a simple example of using the cookies API in the examples/api/cookies directory. For other examples and for help in viewing the source code, see Samples.