I have this problem too. I believe the solution is something like. Uninstall JNGP, turn off your antivirus, download JNGP, put it on the exceptions list, setup JNGP, and turn antivirus back on... I haven't gone through the trouble yet but that is my understanding on what I am supposed to do.
Apparently if you download it with antivirus on (at least certain ones) it will disable the edit tileset extension because it "hacks" the world editor or something.