The Windows version of The Salsa Rhythm Machine is shown here, however, the Mac version is identical. This is a very quick demo so that you can get an idea of what the program looks like:
Here is a screenshot (click to enlarge):
To learn rhythm, you must first learn to count with the music as the musicians do. The Salsa Rhythm Machine shows you the count as it plays music. The color of the appropriate number turns red at the appropriate time. To further help you identify beats, the program will also sometimes display a helpful hint next to the number. In this example, The One is being emphasized by the cymbal.
The Salsa Rhythm Machine shows you how to move your feet in the basic pattern. By following along, you will instantly know what it feels like to be on the rhythm. You need to develop this feeling since it is how you know whether or not you are on the rhythm while dancing. You should be counting while dancing, but sometimes you lose track and have to rely solely on the feeling before you can pick up the count again.
The Salsa Rhythm Machine will give advice, such as above where it tells you to pay attention to the piano.
Here is a demo of the Rhythm Tester which can be used without registering the program: