There are a few different ways to add meal choices to your event.
Create Detailed Meal Choices and Include in Registration.
This is the most "fully featured" approach. You can create your different meal options, display them on your website (optional) then connect them to your registration item.
Have a look here to set your food menu and connect to a registration item.
This will automatically include the meal choices and descriptions within the registration item description.
Add Meal Choices as Sub-Classes
Within your Registration Item section, create a Simple Registration item or an Enhanced Registration item. Within your registration item details, select the Sub-Classes tab on the left.
From the dropdown, to the right of Choose Sub-Category Type choose "Meal Choice" (or Option, etc). Click + Add New Option to create your meal choices. Set them to active.
Once the registrant selects the registration item, they will be asked to choose their option.