The Concept 2 is a great machine. I own one myself and use to train on the Pete Plan for getting to sub-7 as a lightweight; not an easy thing, I might add. It is a cardio machine. It will help you develop your leg, back, and arm muscles for endurance but will do little for gaining muscle mass. You wouldn't use cycling as your primary workout for gaining muscle mass on your legs, would you?
If you want to develop upper body musculature and mass you have to do resistance training.