Yes I know that this thread is old, but this was the one reason I signed up for this forum.
Inherited a D300 with the selector switch broken off. Ordered a replacement online, remove the two screws that removed the plate which also held the lock ring in place.
Used some watch repair tools to pick at the remaing plastic stuck in the brass ball, got it to a point where I just yanked it out along with the brass coupling ring under it.
Removed all the old broken plastic, put the coupling rings (there are 2, one is more fragile than the other) back in and then the ball. yeah, it's hard but go easy and it pops in.
Put the lock ring and metal bracket back in and then the new selector button. Had to adjust it a couple of times to make sure the couple ring holes were visible under the brass ball and pressed the button into.
Pressed it a few more times to make sure it was properly seated and got it worked again.
This seemed like an easy fix but could have gone wrong. But the camera itself was free and a $10 part gave me a second body to use otherwise I would have just sent it into Nikon.