TreeViewer fSourceTree;
TreeViewer fTargetTree;

public void onButtonMoveToTarget()
{
	Object sourceElem = getSelectedSourceElement();
	Object targetElem = getSelectedTargetElement();

	/* remove from source */
	fSourceTree.remove(sourceElem);
	
	/* add to target */
	fTargetTree.add(targetElem, sourceElem); /* here you may like to make a copy of sourceElem by
						a deepCopy instead to avoid unforeseen effects if any */
}

private Object getSelectedSourceElement()
{
	IStructuredSelection selection = (IStructuredSelection)(fSourceTree.getSelection());
	if( selection == null || selection.isEmpty() ) return null;
				
	
	return selection.getFirstElement();
}


private Object getSelectedTargetElement()
{
	IStructuredSelection selection = (IStructuredSelection)(fTargetTree.getSelection());
	if( selection == null || selection.isEmpty() ) 
		return fTargetTree.getInput(); /* return root as the selected element 
						if nothing is added yet*/				
	
	return selection.getFirstElement();
}