The Mustang IV & V amplifiers can accept stereo or mono effects devices. The Mustang III amplifier only accepts mono effects devices. Note that a mono-in, mono-out effect unit will use one send and one return, and only effect one side of the stereo image when used without appropriate adaptors. Also, a "Y" adaptor cable may be used to connect a stereo device into the Mustang III's mono effects loop.
Connect the amp's FX SEND jack(s) to the input(s) on your external effects device and connect the amp's FX RETURN jack(s) to the output(s) on your effects device. Additionally, you can use the Effects Send as a Line Output for recording puposes or a live sound application.