I think you may need to do more study and learn more of not only the .NET API, but the AutoCAD one as well. The 'Curves' class that you mention, is in fact the base class of a lot of AutoCAD entities, with the humble Line being one of them.
If you look up Autodesk.AutoCAD.DatabaseServices.Line in the ARX documentation, you will see it wraps the native ARX AcDbLine class. If you were to look at that, you will see it inherits from AcDbCurve, which in turn is wrapped in AutoCAD's .NET API as Curve I believe.
Therefore, Line inherits from Curve and get's it's methods/properties, with GetPointAtDist being one of them.
Observe:
using System;
using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Geometry;
using Autodesk.AutoCAD.Runtime;
using acadApp = Autodesk.AutoCAD.ApplicationServices.Application;
namespace ClassLibrary1
{
public class Class1
{
public Class1() { }
[CommandMethod("SurveyRand")]
public static void SurveyRandCommand()
{
using (Line tempLine = new Line(new Point3d(0, 0, 0), new Point3d(100, 100, 10)))
{
Point3d closestPoint = tempLine.GetPointAtDist(10);
Document doc = acadApp.DocumentManager.MdiActiveDocument;
Editor ed = doc.Editor;
ed.WriteMessage("{0}Closest point at distance of 10: {1}", Environment.NewLine, closestPoint);
}
}
}
}