The goal with MealPlan is that your grocery list takes care of itself. You just think about what meals you want on what days, and the grocery list will automatically be updated to show the items you need.
For this to work, your meals need to have a list of groceries associated with them. It's easy to see which meals don't, because when you're looking at the grocery list, a small triangular warning indicator will appear to let you know that you've got recipes in the meal plan that don't have any groceries associated with them. Tap the indicator to see those recipes. Add groceries to them, and the warning indicator will disappear.
You can also add items to the grocery list yourself. Tap the + button at the top right corner of the grocery list to add an item, like cat food, that you know you need to get, but that won't show up on your meal plan.
Stores and Aisles
The grocery list divides your groceries up by aisles.
In the Grocery List Settings, you can edit a list of grocery stores you visit, and you can choose your current grocery store. Each grocery store has a list of aisles, and the aisles are sorted in a specific order that you create.
So, for example, if you sometimes shop at one store, which doesn't have a pharmacy, but also shop at a different store, which does have a pharmacy, then picking your current store will either hide or show the pharmacy items depending on how you have the aisles configured. And if you organize the order of the aisles for each store to match the store's layout, then the order of the items on your grocery list will result in a quick path through the store.
A shortcut to picking an aisle for a grocery is to tap and hold it for a second to bring up the aisles list.
The grocery list shows the items you need for the time period you configure in settings. Tap the gear icon at the top of the grocery list to see the grocery list's settings.
The grocery list is built automatically using your planned meals, but you may not want to buy items too far in advance, or may want to start at a particular day. In the Date Range section, pick the starting day, and the number of days to include. The default is the next 7 days.
Tap the share button at the bottom of the grocery list (on iPad) or in the Grocery List Settings (iPhone) to print, email, or send your grocery list to the Reminders app.
Items not showing up?
There are two reasons items you expect to see on the grocery list might not be showing up.
One is the Stores and Aisles feature, mentioned above. If the store you've got selected has an aisle turned off, any items in that aisle won't show up on the grocery list.
The other is that the item has been marked off, either during a recent shopping trip (since you added the item to your meal plan), or accidentally. You can restore these items to the grocery list by using the Clear the Grocery List item in Settings. This resets your grocery list to just the items you need for the meals planned in the next 7 days.