My toilet is leaking from the top?

I just moved into a new apartment, and one of the toilets is leaking when flushed. It is coming out of the tank from the top, under the lid. It only happens when filling up after flushing. I lifted the lid off, and noticed that the little inflatable ball thing-y that is usually there, isnt there anymore. Is this the only thing causing the problem? Is this an easy fix? I don't want to become the girl who calls the maintenance guy for stupid things I can solve myself, so if this is something I can fix myself I'd love to do it. Thanks in advance!