Expression Blend 4 makes invoking a Command, drag-and-drop simple. Working in Expression Blend 4, I have a project with a "MyCommand.cs", "MyViewModel.cs" (both from the Using Commands and MVVM blog project) and an unbound button on the main form:
In Blend, click on the Assets tab and select the Behaviors item in the list. The list contains both "Actions" and "Behaviors".
Actions vs. Behaviors
Silverlight differentiates between "Actions" and "Behaviors". Actions use "Triggers" (e.g. events, storyboard state changes...) to initiate commands, property changes or some other outcome. For example, a mouse Click event can trigger a command execution. "Behaviors" are similar to ...