There doesn\'t seem to be a .Show() kind of method for the Windows.Control.ToolTip, incl in ToolTipService.
Is showing a tooltip what you really want to do. A tooltip has a clear meaning to most users and an expectation that it goes away when moving the mouse (and can come back when you hover over the item in question).
If your aim is to draw attention to something, have you considered some form of floating box which is fully under your control, WPF makes this easy!