Thanks Draknahr and Littleboy. I now understand that either the Impulse account's e-mail address or the Impulse nick can be used. I was only employing the e-mail address because I did not know where to define an Impulse nick (reply #14 giving me, at last, the answer).
P.S. I feel stupid. I already had an Impulse nick : the same one I used on this forum (CyberMage). I never had the elementary curiosity of trying it to log-in instead of my full e-mail address. Sometimes, we miss the obvious. Problem solved.