First thing to check is to make sure that when you go to add grains, make sure that the box next to the 'search' labeled as 'show only items in inventory' is unchecked.
Next, you can download the 'add-ons' for the Simpsons malts to bring these malts into your grain profiles. You can do this by clicking on 'add-ons', then 'add' on the green plus sign in the pop up box. This will give you another pop up box with the selection of available profiles, recipes, and ingredients that you can add to BeerSmith. Scroll down to the 'Simpsons malts' and click on it to highlight it. Then click on 'install add-on' to download it into your BeerSmith folder. The malts should automatically appear in your 'grain' ingredient list when you go to 'add grain' to your recipe.