cannot select an edge to become an outlet boundary

Hi!
After struggling for quite a while with 4.2a, I haven't been able to enable one edge to become an outlet. What I did was to construct two objects: the first one is a sequence of lines, which I then made to become a single curve, which in turn I converted into a solid. Then I built a rectangle, which is supposed to embed the first flat, 2D figure. The first figure has an inlet, which was easy to make, just selecting the proper boundary, and that was it. The outlet, no matter how many times I selected that boundary, won't be added to the right blank place, so that I can declare the outlet properties of it. I gave a look, and it happens that most edges (or flat boundaries) are walls. I tried to build the two objects in several ways, and I still haven't figured out how to make them, so that I can take this boundary as an outlet.

This is a matter of some urgency, so please, please, if you have a clear idea of what I need, do write it down. Or even if you only have a hint, but has some algorithmic clarity. I am including the geometry

Yours

Arturo