There have been a few question on how to use analog inputs on the Netduino Plus and Netduino Plus 2.
I hope the following two posts help to clear up some questions.
Post#2 in this thread will have an example using the: Netduino Plus.
Post#3 in this thread will have an example using the: Netduino Plus 2.
For each board, I also added a zip of the project and files.
Good Luck!