Hi psk_12,
Plus symbol in top right is Quick Create button, which is used to create records with quick create form.
If you want to entity appear in the list when you click it, you need fill 3 point:
1. Navigate to Customization Customize the System and look for the entity and check if "Allow quick create" is checked.
2. Check in the forms if the "Quick create" form Exist in the Entity you are trying to add.
3.Add the entity and forms in Model driven App which you are trying to access
Result:
Or you can refer following document:
Add a table to the quick create menu in Dynamics 365 App for Outlook (Dynamics 365 apps) | Microsoft Docs
