This is networking 101 kind of stuff. Are the machines hanging off a hub or is it a switched network? Have you actually done any reading about ARP or did you just think you'd click the big red "poison it now" button without any clue as to what that might do?What's even stranger (to my limited knowledge) is that Wireshark only sees packets to and from the AP to and from the computer it's running on... Packets for other computers don't even show up in Wireshark.
If you continue to "play" with things without a little background knowledge you're just going to end up wasting time. Yours, whoever owns the network, whoever maintains the network, whoever else has systems on the network etc which in turn means wasted money, etc.
Yes I know "everyone has to start somewhere", I'm simply suggesting that you start in a place where you get the necessary background knowledge on the things you're playing with before causing someone (yourself or others) problems due to not understanding how and why what you're doing may impact things.