Have a look at this
'modified from the ACAD 2006 Helpfiles
Option Explicit
Public Sub drawSpline()
Dim splineObj As AcadSpline
Dim startTan(0 To 2) As Double
Dim endTan(0 To 2) As Double
Dim fitPoints(0 To 8) As Double
startTan(0) = 0.5: startTan(1) = 0.5: startTan(2) = 0
endTan(0) = 0.5: endTan(1) = 0.5: endTan(2) = 0
fitPoints(0) = 1: fitPoints(1) = 1: fitPoints(2) = 0
fitPoints(3) = 5: fitPoints(4) = 5: fitPoints(5) = 0
fitPoints(6) = 10: fitPoints(7) = 0: fitPoints(8) = 0
Set splineObj = ThisDrawing.ModelSpace.AddSpline(fitPoints, startTan, endTan)
ZoomAll
End Sub