The HorizonalContentAlignment can be used to right-align text *within* the label. For example:
c1Label1.AutoSize = AutoSize.Height;
c1Label1.Width = 500;
c1Label1.HorizontalContentAlignment = HorizontalAlignment.Right;
c1Label1.Text = "right...";
The HorzontalAlignment property will align the label control itself within a parent StackPanel (not a C1StackPanel).
Neither property will affect the alignment of the control within a Canvas element though. You are supposed to position elements within the canvas yourself.
Hope this helps.