In order to create executable instances of a org.eclipse.jubula.communication.CAP (Test Step) you have to use the corresponding factory methods available for each toolkit and pass a valid and fitting ComponentIdentifier:
org.eclipse.jubula.toolkit.javafx.JavafxComponents - see e.g. the Figure 2.4, “JavaFX-Toolkit component factory usage to create an instance for a CheckBox”
org.eclipse.jubula.toolkit.swing.SwingComponents
org.eclipse.jubula.toolkit.html.HtmlComponents
org.eclipse.jubula.toolkit.swt.SwtComponents
On these UI component placeholder instances you can call methods that will return an executable CAP that, when being executed on the AUT connection, performs the specified test step: