These are the steps you need to do to get more permisions for your development access token:
- Go to the following URL: 'https://developers.facebook.com/tools/explorer/'
- From the dropdown near the top, select the application you want to get extended permissions for
- Click on 'Get Acces Token'
- Select all the permisions you want (including publish_actions)
- Copy the access token generated in the text field besides it, and use it inside Unity when it prompts you for an access token.
Hope this sorts it out for you. If any step is not clear, let me know