Hello and welcome,
I think I have an answer to your problem, but you might want to check out the "Create 2D mesh" function in the AC3D help file. Seems like that will do exactly what you want without you having to create new polygons manually. Make sure you do the part in the help file that tells you how to remove the lines and keep the vertices, since the lines may cause you problems being on top of those new polygons later.
As far as your original question, not sure if you're onto this, but when you're nearsnapping, it'll only work of the polygon is near the existing vertex in all 3 dimensions --- you may be right on top of a vertex in the xy plane, for example, but the Z location of the new point you're placing must also be near to the existing vertex's Z position. To correct this, you can move your mouse cursor to a different view during polygon creation to get the next point to line up correctly.
Also, when starting a polygon, notice the XYZ coordinates in the upper left of the AC3D window (the top of the control panel). It will show you the coordinate of the point you're about to place. If you're placing a point on the XY plane, the Z plane will probably be set to zero - that's because the 3D cursor is at 0,0,0, and AC3D places new objects/points at the 3D cursor when there's no other reference point. To fix this, you can set the 3D cursor using "Tools | Position 3D cursor".
There are other tools like "create ordered surface" and "create convex surface/object" that may help you, but it seems "create 2d mesh" is the way to go.
Hope this addresses the problems you're having.
Dennis
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