Hi jesper - it looks like what you have there is an "edge" curve - which is a curve that is part of a face trimming boundary.

You can't add points to edge curves, you can only add points to "standalone" curves - curves that are just plain curves and not ones that are making up the trim boundary of a face.

You could add points to the top surface though (the points you see on the box are the points of the "underlying surfaces" of each face), or you could extract the edge curve by selecting it and doing copy and then paste, to get a standalone curve which you could then edit directly.

It kind of looks from your screenshot that you may be trying to use a polygon modeling point sculpting type approach - in general MoI is not focused on that type of modeling method, instead of making a starting block and then pushing points of it around it's more typical in MoI to use a much different "construction" oriented approach where you draw key profile curves and use those to build pieces and also cut pieces. So usually you would do something more like build curves to form profiles for your twisted shape and then build it by lofting, extruding, cutting, etc... so that once you have it as a solid it's in the desired form at that time already. As opposed to starting with some other kind of solid and then trying to mutate it by squishing individual points of the solid around.

You might want to check out some of these tips for people coming from a poly modeling background:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4865.2

- Michael