If you have installed Diagramming for WPF distribution locally and your project targets .NET 4, you should be able to see controls from the API in Visual Studio toolbox and can start using them by dragging to Xaml UI. If targeting CoreCLR (.NET 5+), locally installed assemblies do not show in the toolbox at this time, and you must add references to them manually in order to use the diagramming API. To do so, select the Add Project Reference command from Project menu:

In Reference Manager, select the Browse tab and click the Browse button. Navigate to the Diagramming for WPF install location, select assemblies for respective .NET version and click Add:

Once assembly references have been added, you should be able to add controls from the library to your UI (Diagram, NodeListView, Overview, etc).