In order to display items, you have to populate the RadListBox control with some data. You can do this in two ways:
Manually, by adding items to the Items collection.
With data binding, by using the ItemsSource property.
You can't have items added manually to the Items collection and set the ItemsSource at the same time.
In this section you will find:
Populating RadListBox declaratively - explains how to populate your RadListBox control with some static data in XAML.
Populating RadListBox in the code-behind - explains how to populate your RadListBox control with some static data in the code-behind.
DataBinding Support Overview - here you will find information about the supported data sources and RadListBox's data binding basics.
Binding to Object - explains how to bind your RadListBox to a collection of objects.