by Tim Hunt.
So, look at https://github.com/davosmith/moodle-navbuttons. You will see that there is only a master branch and a MOODLE_19_STABLE branch, so presumably you need to install the master branch if you are any Moodle 2.x. The readme file there confirms that hypothesis, although it could be more explicity.
So, just do a git clone of that repository.