Okay. Find presets manager (it is in the Edit drop down menu on my Photoshop) >Patterns. Click the little arrow at the side for a list of patterns, choose the one you want, the press the load button and exit.

Or, which is easier, click the pattern tool in Toolbox and a menu bar should appear along the top, with the pattern loaded shown in the little thumbnail. If you want to change it click the little arrow at the side, as you would do for brushes, and choose from the drop down menu.
I'm not that good at explaining so I hope that was at least of a little help.