Hi simba, the best way to get a smooth closed end with loft is to use the option Loft style = "Loose" inside the Loft command and also include a point object that is in the same plane as the end profile.

I've attached an example 3DM model file here - when opening the file notice how there is a point object at either end, that will be the closing point. Also notice that the point object is in the same plane as the first or last profile curve as well.

Then to do the loft, select everything and run Construct > Loft, and change Loft style to "Loose":



The "Loose" loft style makes a more relaxed surface which follows the general shape of the profile curves but does not force the surface to go exactly through them, it works well for making smoother shapes particularly ones with smooth closed ends when used with a point object for the final start or end position. You create point objects under Draw curve > More > Point. To make a smooth end put the point object in the same plane as the last profile curve before the end, that will make the surface be tangent to that same plane in the loose loft result.

Some other examples of loft to a smooth closed end here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6082.10
http://moi3d.com/forum/index.php?webtag=MOI&msg=6082.8

In the attached file you'll get a result like this:




- Michael