From the left sidebar of the Planner you can click on the recipe name to see the recipe details.

To the right of the recipe name is the handle icon. If you click and hold the handle you will be able to drag the recipe onto the Planner and drop it to a day and mealtime.

On a tablet will need to tap and hold on the handle icon with some intention, so your tablet recognizes you are wanting to drag the recipe instead of scroll the recipe list.

How do I drag recipes on an iPad
I can't figure out how to hold it so that I can drag it.