The dx100 package was merged into the current motoman_driver package, quite some time ago already (notice the groovy version specifier in the link you include). Most of the functionality was kept in the motoman_driver version of MotoROS (the MotoPlus application that is part of motoman_driver).
Can you clarify what it is that you are trying to do? The .c files in MotoROS are not usable outside the MotoROS runtime environment, so they can’t be used in a plain (ROS) application.