When you are viewing a recipe, click on the link "Add to meal planner".
Then choose when you want to eat that meal (date , Breakfast / Lunch / Snacks / Dinner ) and click OK.
If you want to add a recipe in the meal planner, another option is to search a recipe in the sidebar in the meal planner page, and drag it in the calendar.
If you want to change the date, just select the recipe in the calendar and drag it to another day.
If you want to put you recipe in the dinner section, select the day view, then select the recipe and drag it to the dinner section.