How to make a circuit to run DFRobot with a Netduino
There are a couple of issues with the proposed design:


The 4n35 is an optocoupler, not a regular transistor. While the output side can conceptually be treated as a BJT, the maximum collector current rating for it is 100 mA, which may be insufficient for driving multiple motors, or even one motor, depending on the motor's current requirement. 


For the switching  element, use a Bipolar Junction Transistor (BJT) such as the 2n2222 (800 mA maximum continuous collector current) or a logic-level MOSFET such as the IRLML2502 (3.4A @ 4.5 Vgs, at least 2 Amperes at 3.3 Vgs) instead. Depending on the motor current requirement, and the number of motors to be run, you may need a higher current rated BJT or MOSFET.

The NetDuino IO pins output 3.3 Volts, with an absolute-maximum current rating of 25 mA per pin, 125 mA total. Even if the 4n35 is to be used for optical isolation, which is its purpose, adding an indicator LED in series with the input of the 4n35 is not a good plan, as the voltage drop for the LED (e.g. 2.2 to 3.4 Volts) will leave too little headroom for the 4n35 internal IR LED (~1.2 to 1.5 Volts). 


Using the LED in parallel with the input too isn't a good idea, since the current for the 4n35 (e.g. 20 mA) and for the LED (e.g. 10 mA) will add up as the load on the IO pin, thus loading the Netduino's GPIO pin beyond its rating. 
In other words, either drive your indicator LED from a separate GPIO pin, or move it to the output side. 





A high-level solution, leaving you to work out the specifics:







simulate this circuit &ndash; Schematic created using CircuitLab


(optional for isolation) Drive the 4n35 input from the GPIO pin with a series resistor to limit GPIO current to under 20 mA
Use the output to drive a MOSFET (or use a BJT with appropriate biasing if desired)
MOSFET (or BJT) as switch to power the motor(s). 


Note that the motor will be on when the netduino output is low, and vice versa. The circuit can be reshuffled to eliminate this inverting of logic, if required.