What could it be the problem ?
Anything
OK. For adding new field to Customer follow these steps:
1. Open Domain Layer > Samba.Domain > Models > Customers > Customer.cs
2. Add a string property named PostCode.
3. Open Presentation Layer > Samba.Modules.CustomerModule > CustomerEditorViewModel.cs
4. Add a string property for PostCode filed and update get;set; methods. You can check other properties (for example Address) for hint.
5. Add a textbox and bind it to PostCode property on CustomerEditorView.xaml for editing PostCode.
Customer.cs contains Customer Model. SambaPOS automatically generates database from these models. You can either delete database for regeneration or add PostCode field manually to Customers table.
CustomerEditorViewModel sits between Customer Model and CustomerEditorView. It publishes model properties for editing / updating and supplies additional data for editor. For example Group Code combox values binded to ViewModel's GroupCodes property.
If you are interested you can read about MVVM pattern for more information.