Pins 1 and 2 are connected together, and need to be supplied with the same voltage as what you are switching. Pin #4 controls the MOSFET on the left when grounded, and pin #3 controls the MOSFET on the right when grounded.
The Fusion Brain works with switched grounds. So when you connect the FB to the MOSFET board, connect the + side to the "bottom" pin and the - side to the "top" pin. The drains of the MOSFET are on the outside bottom 4 pins, and the sources are the middle two.
For best operation, the sources should be tied to ground, (it needs to be the same ground as the Fusion Brain). Then your load gets power normally, and the ground of your load is connected to the drain pin on the MOSFET. So when the Fusion Brain turns on an output, the pin is grounded, and the MOSFET allows current to flow through the power source, to the load, to the MOSFET, and to ground completing the circuit and turning on the load.
Please note: This is a hobbyist grade product. As such, limited or no documentation is included with it and advanced technical skill may be required to use it. Help may be available on the mp3Car forums or by contacting technical support at our store. However, as manufacturers of hobbyist grade products usually create them in their spare time, response time to technical questions can sometimes take extended periods of time. By purchasing this product, you acknowledge these disclaimers.