Usage: goodfet.mcpcan verb [objects] goodfet.mcpcan info goodfet.mcpcan test goodfet.mcpcan peek [adr] goodfet.mcpcan sniff [kHz] goodfet.mcpcan isniff [kHz]
The MCP CAN chips are wired directly into the GoodThopter11 and similar boards. See their wiring descriptions for details.
An article on using the GoodThopter boards with various automobiles is in the works. It will appear on Travis Goodspeed's Blog in due time.
Until it is available, wire your GoodThopter into a car's OBD-II port and sniff at different rates until traffic appears.