From the top menu, select 'Recipes' > 'Inventory' and this will open up your current inventory of ingredients (everything). You can select along the ribbon menu to add hops, fermentable, yeast, misc, and even water which will bring up the profiles for that category. Start typing the name in the search box and your list will pare down based upon your entry to a limited number of ingredients. You can select the one that you want as soon as it appears on the screen, enter the amount you purchased and the cost per unit and then select 'OK to save it to your inventory.
Now when you use that item in your recipe, you can hit the 'remove inventory' button to have the program deduct the recipe amount from your inventory.