Fortunately, you can order new rubber button thingies. In order to put the new one in, you have to pull the stem from the broken one. Tweezers and needlenose pliers are handy for this job. Here is a crappy pic of a new button and the broken pieces of the old one:
Then all you have to do is push the new one in the hole and you have nice new rubber button thingies.
Tada!
Ciao