Google Now is simply part of Google Search. Other ways to access it:
1. Tap the persistent Google Search bar at the top of a home screen (assuming that you haven't disabled it by using an alternate launcher).
2. Place the Google Search widget on a home screen and tap on it.
3. Tap the Google Search icon in the app drawer (it's marked simply as "Google" and shows as a stylized "g" on a blue background). Alternatively, place that icon on a home screen as a shortcut.
Keep in mind that if you tap on either the persistent Google Search bar or the Google Search widget, the nice graphic background design at the top of the screen is replaced a list of your most recent searches. If you drag up from the home button or tap the Google Search icon instead, the background graphic is displayed. But as soon as you tap inside the search box, the cute background will slide up and away, to be replaced by your recent search list.