Wireshark / tcpdump let you dump data on the access point you are connected to. To dump all WiFi data on OS X, you can instead:
sudo /usr/libexec/airportd en1 sniff 11
which will create a pcap file in /tmp with a random looking name. You can open it with Wireshark / tcpdump or any pcap compatible program.
- Adjust "en1" to your airport device name.