Is there any way to get the information about admin who installed my app?

Question

I am developing an application for the Google Apps Marketplace.

I use the Licensing API to retrieve the notifications. I need to get the information about admin, who installed the app, but the API doesn't provide me with this.

The solution I found is to use the Directory API to search users, but I get an error

{
    "error": {
     "errors": [
      {
       "domain": "global",
       "reason": "forbidden",
       "message": "Not Authorized to access this resource/api"
      }
     ],
     "code": 403,
     "message": "Not Authorized to access this resource/api"
    }
}

What is wrong with my request? Are there any other solutions to get the information about admin?

The auth scopes:

- "email"
- "profile"
- "https://www.googleapis.com/auth/drive"
- "https://www.googleapis.com/auth/drive.file"
- "https://www.googleapis.com/auth/admin.directory.user.readonly"
- "https://www.googleapis.com/auth/admin.directory.group.readonly"