Download the xml file to your computer. Then log in to your admin control panel and open the "Plugins & Products" section on the left (it's near the bottom) and click on "Manage Products". Next click on the "Add/Import Product" link under the product list and press the "Browse" button. Select the xml file you saved in the first step and press "open".
That should do it - read the mod page to see if there's anything else you need to know to use it.