It's likely that the model is from a version later than ROS indigo. This issue could be in URDFLoader or in the Xacro Parser. First step is processing the xacro files to URDF using the original ROS xacro converter to see what the result is. Use the official, non modified version xacro converter if the URDF model still doesn't work correctly.
Model from this repository does not load properly. Specifically this xacro root.
It's likely that the model is from a version later than ROS indigo. This issue could be in URDFLoader or in the Xacro Parser. First step is processing the xacro files to URDF using the original ROS xacro converter to see what the result is. Use the official, non modified version xacro converter if the URDF model still doesn't work correctly.