Normally, you connect to an AR.Drone using its Wi-Fi interface, but if for some reason this is not working, you can open a serial console to do any necessary fixes or simply have a permanent connection to the device for debugging, for example. To open a serial console, you will need a USB to UART connector, a USB extension (optional) and some wires. Although there might be other USB to UART bridges that would work fine, I have only used the CP2102 bridge from Sillicon Labs (Amazon, $6.97).

(USE THESE INSTRUCTIONS AT YOUR OWN RISK)