1. Use the selection tool select all areas you wish you add something new to, in this case a pattern.
2. Then you’ll want to click on the materials menu (colour palette – top right in default setting I think), and browse through the tabs: ‘Color’, ‘Gradient’, ‘Pattern’. (screen shot)
3. Pick the pattern you like, then use the flood fill tool (bucket) and click in those selected areas.
Tell me if that’s too vague and I’ll take more screen shots. 🙂