Hi ska234 - it kind of looks like you're currently trying to draw an 2D outline curve that's not what is called a "plan" curve (a blue-print like view of the Top or side of a shape), but rather you're trying to sketch what is a 3D angled view of the harp.

You can't really construct a 3D shape from a 2D curve traced that was traced only from a 3D angled view, you need to construct the main curves from "Plan" blueprint views instead...

Also like Max writes above you probably will want to go through somewhat of a carving process - some areas of your model will be formed from some larger piece of stock material (formed from an extrusion for example by drawing a plan curve and using Construct > Extrude), and then some booleans will be done on that object to carve away parts of that stock material, rather than trying to draw everything directly in its final place right from the start.

Your harp is unfortunately a fairly advanced type shape to work with in a CAD program, I would encourage you to spend some time working on more simple shapes where they are more fully defined by their plan views first to gain familiarity with how MoI functions for a while before trying to tackle a more difficult project like the harp.

Try going over the introductory tutorials here first:
http://moi3d.com/3.0/docs/tutorials.htm

- Michael