Are you doing this from an app? If so that doesn't work. It has to be done from a web browser. It is also not the same as "mobile ordering" so it won't show up if you select that option. It also only works for breakfast and lunch.
If you are within 30 days of your reservation, go to MDE in a web browser, locate your reservation and select the option to preorder. If you are not within 30 days you won't see it.