If on Windows, using Alt and the right numpad is usually the way to do it. In GNOME, I use Control-Alt-Shift-U to key in a Unicode codepoint using the numrow (I do not have a numpad, being on a laptop).
To my knowledge Unicode (UTF8) text can be used in SVG or in an Inkscape text object at any time, so I do not believe one has to enter a 'Unicode mode' to enter such characters, but perhaps you (John) meant the OS interface to specify a codepoint and have the resulting character entered as if typed by a keyboard.
-Arlo James Barnes