It sounds like you are trying to edit the files that a particular package has deployed. This is not a sustainable way for doing development, so if you intend to do more than a quick hack I would recommend doing it 'properly'.

Instead, you should setup a development environment following the instructions here: http://docs.openstack.org/developer/horizon/quickstart.html and read the documentation on customising Horizon: http://docs.openstack.org/developer/horizon/topics/customizing.html

To work with Horizon, you should first learn how to use django. The tutorial on their website is excellent.