By default you can add snacks to your meal plan by putting them into the "Snack" slot for each day:
MealGarden let's you create your custom snack schedule within a meal plan. To use it check the "Schedule detailed snacks" checkbox within meal plan edit screen and specify how many snacks after each main meal, but before the next main meal should your meal plan have:
Important: Do not forget to click "Save" after you make the change.
Once this is done when you click on "Edit Schedule" button the Edit Schedule screen will reflect your snack schedule:
Now just add meals to those slots like you do for any other part of the meal plan.