Have you added the content to the Content folder of your project and set the Build Action to Content (in the properties window).
I'm not exactly sure what the process is to use assets in the .xnb format. I know it can be done that way but I normally just add the raw image and sounds files to my Content folder directly. Font's might be a little trickier to do that way though.
EDIT: 2015
A lot of things have changed since this question was written. These days MonoGame has it's own Pipeline tool for processing content into XNB files. The first thing to try would be to process your SpriteFont
using the new Pipeline. This way you can avoid the need to depend on XNA altogether.
Alternately, another way to get around font issues in MonoGame is to pre-render them to a texture using the BMFont tool and use MonoGame.Extended to render them in your game.
Once you've installed MonoGame.Extended you can load fonts created with BMFont just as you would a SpriteFont
but using the BitmapFont
class instead.
_bitmapFont = Content.Load<BitmapFont>("my-font");
Then render some text like so:
_spriteBatch.Begin();
_spriteBatch.DrawString(_bitmapFont, "Hello World", new Vector2(100, 200), Color.Red);
_spriteBatch.End();
I have a full tutorial on my blog