After you have marked and referenced a Curve or an Edge and it does not appear in the 3D Viewer (i.e. its 3D position is not being calculated) then some of the requirements for Curves or Edges are not being met.
Note that the 3D status of an Edge or Curve is shown in the Status Bars when these objects are selected. See the Status Bars sections: "Curve - open Status Bar" and "Edge Status Bar".
Curves must be marked and referenced on two or more oriented photographs. Select the Curve and look at the number of oriented photographs listed on its Status Bars.
If an Edge is constrained in a single photo project it can become 3D, otherwise Edges must be marked and referenced on two or more oriented photographs. Select the Edge and look at the number of oriented photographs listed on its status bar. In the multi-photo case also study the angle shown in the Edge's status bar. If the value is too low (usually less than 10 degrees) then the Edge calculation will not work and the Edge will not become 3D.
You should study what photographs these marks appear on and whether they are suitable for that Edge or Curve. See the Quick Start Guide section Photography for Curves, Edges and Cylinders for more information.
Edges and relatively straight Curves can be modeled sufficiently by marking them on two photographs if those photographs rotate around the axis of the object. This is also described in the Quick Start Guide section - Photography for Curves, Edges and Cylinders.
Curves, on the other hand, if they are closed curves or change direction considerably will often have to be marked on three or more photographs. If you have a curve where only part of the curve appears in 3D this is most likely the problem. See the next section.
Another consideration with closed curves is that PhotoModeler assumes all marks "view" the curve from the same side. Imagine a horizontal metal ring that you will be modeling using a closed NURBS curve. If you photograph the ring from only one side (call it the upper side) then you will not encounter any problem. On the other hand, if you photograph the horizontal ring from below also and mark the ring on both the view-from-above and view-from-below photos PhotoModeler will not be able to disambiguate the orientation of the ring. The resulting 3D closed curve will lie in the wrong plane and may have the wrong shape. All closed curves must be marked on photographs that view the curve from the same side. See the Curve tutorial for an example.