Setting this property will not automatically create a model for you, if Diagram.Model is null. You will need to set that Diagram.Model property explicitly, usually in code. When the value of this property changes, the new value is used as the new value for the Northwoods.GoXam.Model.IDiagramModel.NodesSource property.

When the Diagram.Model is replaced, or when the model's Northwoods.GoXam.Model.IDiagramModel.NodesSource changes, this dependency property will automatically be set to the new collection value.

If you want to data-bind this property, you should use "Mode=TwoWay".

Reference