A voltage divider is the way to go on this one. Here's a link to the theory behind the voltage divider and a handy calculator:
http://hyperphysics....ric/voldiv.html
Also, this is a pretty good voltage divider calculator:
http://www.sengpiela...tagedivider.htm
Use calculator # 2.
According to the datasheet, the input resistance of the ADC is maximum 50k Ohms (If I read it right) so that would have to be taken into account for your measurements to be accurate. The input resistance would be represented in a voltage divider as RL.
You can mess around with the calculator on that page and try different values. Best to do your calculations to get 3V or 3.3V as an open circuit voltage and hook it to the Netduino. The voltage will then drop to around the value given for the loaded voltage. You can then adjust the voltages as necessary. Remember to disconnect the voltage divider from the Netduino when changing out resistors. You don't want to brick your analog port.