In order to buy Ground Pounders - High Rez Pack, select the offer that suits you from the list. Click on it, after which you will be taken to the store website where you can buy Ground Pounders - High Rez Pack.
This is a free, optional high resolution texture pack for Ground Pounders. You don't need it, it just makes things even prettier - it is also a sizable collection of files, which is why it is optional.