How can I add a reference in monodevelop?

I need to work with Json in mono and I'm using the IDE monodevelop.

How can I add as a reference json.net in monodevelop ?

In your project there should be a folder called References. Right click it and choose edit references. Then check the box next to the package you want to add

For monodevelop 4.012 try look in the Solution->Packages->Edit Packages and look for the library you need.