Download the file.
With BeerSmith closed, double click the file and BeerSmith will open automatically. You'll see the full list of grains in the open tab.
Open Ingredients > Malt
Copy the Castle malt list and past it into the Malt tab.
At this point, all of the malts should be in your malt database. Whenyou close BeerSmith, it'll prompt you to save, but you can click No.