You can do that by doing the following:
Find the recipe you want to add to your meal planner.
Below the nutritional information is a link that says "Add This Recipe to Meal Planner."
Click on the link that will open a small calendar. Select the date to which you'd like to add the food.
Select the serving size, the meal you'd like to add it to, and check the box if you'd like to save to your favorites for future use.
Click on "Add Food to Today" and the recipe will now show up in your tracker.