First, download Processing (free).
Second, download both the Arduino and Processing code I've provided here.
Third, upload the Arduino code and run the Processing code.
If you examine the Processing software, you will notice that a number of hot keys have been defined. I won't repeat them here, but please take a look so you know the basic functionality. These hot keys can quickly be edited to your liking. Also, if you're having a hard time getting Processing to select the right com port there is a variable for that (note that this variable is the index in the list of available ports).
Lastly, I've added the ability to output a 2Hz square wave on the Arduino. Connect this pin to your analog input to test the scope. Enjoy!