Here is a quick guide on how to install and use patterns in Photoshop CS and CC.
Start by selecting the paint bucket tool. Then from the dropdown (1.) choose pattern. Open the pattern box (2.) by clicking on the down arrow. Then click on the wheel (3.) and select Load Patterns. A dialogue box will popup and here you can select the pattern (.pat) you want to load.
How to use patterns
Create a new layer and use you paint bucket tool with the pattern option selected as shown above to fill the canvas with the selected pattern.
You can also use the patterns by using the Pattern Overlay layer style option. In order to be able to use the pattern overlay layer style you will need to have some content on your layer, whether it’s solid color, text or a shape.