To start using the Debugger, all you have to do is press the “Play” button in the top left of MonoDevelop’s window:

If you press that, MonoDevelop will prompt you to “Attach to a Process”. Just select your instance of unity and press attach. The dialogue will vanish and you will be left with a “Stop” button instead of a play button.

To start debugging, press your mouse to the far left edge of the editor (next to the line number) and a red dot will appear, you would have just created a breakpoint!

This will not do anything for now, however, if you now go to Unity and press play in your editor window something great will happen…

At the very bottom of the window, if you have the locals window open (if not, Go to View > Debug Windows > Locals), you will see all of the variables that currently exist in the local instance and their values at the time of the breakpoint being hit.

To continue the applications execution, just press the “Play” button in MonoDevelop

Your script will continue its execution (and Unity’s editor will no longer be frozen). Of course in this instance, the script will hit the breakpoint again on the next frame. So just left click the breakpoint in MonoDevelop and hit the Play button again so it doesn’t execute the breakpoint again.