Here's a Step-by-Step for a Mac, which is what I use. You can also probably just Google it, because EA has a step by step process on there website if you'd like to follow it. This one's very general but if you were able to install the base game, you should be able to install the expansion:
1.) You put the disc in your computer.
2.) You follow the installation process for the expansion pack, which should pop up when you double click the cd icon and you begin the installer, just like the base game.
3.) Once you have it installed, play the most recent Expansion you've installed, which in this case, is this one. Don't uninstall the Sims base, though, because all of the data is still on it, but it has been copied into the Expansion Pack for you to play with it. (So play on the Expansion Pack disc from now on, but keep the Sims installed on your computer!)
Hope this helps!