1. Open Wireless Diagnostics located in /System/Library/CoreServices/Applications/

2. Press ⌘Command+6 or choose Window > Packet Sniffer

3. Choose a channel and frequency

4. Click Start

Once you click Stop, a .wcap  file will be added to your Desktop.  You will need a program like Wireshark to read the file.  Wireshark can also capture packets, but it is interesting that Apple has added this function to Wireless Diagnostics.