Here's a couple of tutorials on the general concepts:
http://3drockstar.com/model-a-shark-using-splines/
http://www.erikasorson.com/tutorials...l_splines.html
I'm not sure if that's really the best method for AC3D, however.
AC3D's splines, once created, are locked in place and you can't adjust the node handles. More importantly, a lot of people making spline cages in other programs are using nurbs to connect the splines and create the topology, which AC3D doesn't support.
You can create some--but not all--types of topology from a set of edges in AC3D using the convex hull tool instead, then use the hull as a subdivision cage. (Select your edges, then click Vertex > Create Convex Surface\Object.) You'll most likely need to combine any triangles back to quads to make it smooth before you apply the subdivisions. If you go this route, it's best to select a small section at a time, or the convex shape tool is likely to create a lot of extra surfaces you don't want. For low-poly shapes this works fine, but if you have a lot of surfaces this can be tedious.
There was also a plug-in floating around somewhere (polyfromlines, I think) that would let you generate a surface from edge-lines, but I never tried it so I can't tell you if it works or not.