Probably, You haven't added the hector navigation stack to your ROS_PACKAGE_PATH.
Verify that and if You haven't done it yet, You could download the hector_navigation package to Catkin Workspace. Then, you have to compile the package, in catkin, using de catkin_make command in the catkin directory, not in the catkin source directory because You could damage the catkin workspace. If all is OK in the compilation console, You have to add the catkin workspace to rospath using the command source devel/setup.bash .
After that, You could list all your packages and see your hector_navigation stack with all their packages in the rospack list.
The last step is to build the entire package with rosmake hector_navigation command. If all is OK, you can launch the hector_exploration_node with the following launch file:
<launch>
<node pkg="hector_exploration_node" type="exploration_planner_node" name="hector_exploration_node" output="screen">
<rosparam file="$(find hector_exploration_node)/config/costmap.yaml" command="load" />
</node>
</launch>
In the costmap.yaml you can configure the footprint of your robot.