You've downloaded new pattern and you want to load it to yous Adobe Photoshop, read this tutorial.
go to option palette and change Foreground on Pattern.
Then click on (1) to open the Pattern Picker.
Than click on little triangel in the right upper corner (2).
Next on the list you choose 'Load Patterns'
In the dialog box choose the .pat file you want to load and your new patterns appear in the Pattern Palette and are just ready to use.