How Do I How do I get ying-yang island in my game?

It comes as a zip. Open the zip and put the two pieces into the folder:
(user specific)My Documents\EA GAMES\The Sims 2\SC4 terrains.

Then start your game. When you get to the neighborhood choice screen, click on "Create custom neighborhoods" and then pick the yinyang from the list.

If you don't want to take a chance on messing up, then use one of the others as a trial before trying the downloaded on.

The full directions on how to do this are on page 67 of the little manual which comes with the game (if you can't get it to work using mine.)