I'm using Nitrous.io and cannot directly access package files on my dev server there (not sure if that's specific to them or all Meteor projects).
I'm also using Autoform. I needed to change something in a file in the Autoform package. Since I couldn't access it directly, I created a fork of Autoform and made the changes there.
But... I'm new and clueless as to how I can test this change on my dev server.
Create a packages directory in your meteor project and then "git clone" your fork into it. When you "meteor add" the package, meteor will find it in your packages directory.