To load the Brickstock XML-files, right-click on the link and use “Save target as...”
To open the Brickstorck XML-files, Brickstock-software is required. This can be downloaded at www.brickforge.de . This program makes it easy to find the required parts at Bricklink.
In Brickstock, there is an option “Export Wanted List XML to clipboard”. Use it after loading the parts-list file and press OK. Then the Bricklink site opens on the "Wanted list Mass Upload page". Paste the wanted list to the entry field (Ctrl + V) and press "verify items". Scroll down and press "Upload file" and then "view your updated wanted list"
If you go to the (next) tab "By shop" you can see which shop have how many of the parts you need.