UPDATE 2 (2020-03-16)
The newer Android Studio version changed the location of this button. Now if you want to see the layout design preview you will need to press one of the buttons at the top right of your xml. The button that looks like an image icon will open the design dashboard, while the button next to it will open the split view where the design is placed next to the XML code:
ORIGINAL (2013-05-21)
You should have a Design button next to the Text button under the xml text editor:
Or you can use the Preview button in the upper right corner to add a preview window next to the XML code:
UPDATE:
If you dont have it, then do this: View -> Tool Windows -> Preview
