Just take the drum off and you will notice a hole in the backing plate behind the hub, Line the busted stud up with the hole and give it a wack with a hammer. The studs are just a press fit.
The new stud needs to be slotted in the back then put the nut supplied with the stud on and tighten it up, it should pull through the hub and seat itself.
Make sure you grease the thread before you put the wheel on, because the new studs come without grease and the same thing will happen if you leave it dry.
By the sounds of things, it is probably worthwile putting some grease on all the other studs.