Hi Patrick,
Are you getting an error message? Since the lines are already complexed, are you pressing <Alt> before you start picking them with the Parabola Between Elements command? You don't have to hold it in, just press it and then it lets you pick the separate elements and enter a curve length. Note: In general, I find it easier to use Line Between Points to place the lines if I'm adding Curves 'after the fact' like this, then Complex them. I use Complex by PI if I'm adding the curves as I go.
Sam