The Bone Tool

You will notice that when you added the bones to the arm, all the parts of the arm moved from layer1 to an new armature layer:

armature layer

Click frame 1 of the armature layer, then open the properties window (Ctrl+F3):

Select Runtime instead of the default, Authortime, then run your movie.

select runtime

When you run the movie, the user will be able to drag the arm and reposition the arm.

NEXT: Constraints