I'm kind of guessing here, but it's worth giving it a shot. When you registered your ActiveX Control, I'm assuming you assigned a Program ID along with it, correct? If you did, then you might be able to add it as a shape object to the slide you specify. When you add a shape, you can add specific types of shapes, including OLEObjects.
For example, in the code below, I add an ActiveX ComboBox Control to Slide 1 in the Active Presentation using PowerPoint VBA. The critical thing to note is that the ProgID identifies the object being inserted.
Sub ActiveXControlAdd()
'Declare your variables.
Dim PPTPres As Presentation
Dim PPTSld As Slide
Dim PPTShp As Shape
'Grab the slide you want it on.
Set PPTPres = ActivePresentation
Set PPTSld = PPTPres.Slides(1)
'Add a shape, but make sure it's an OLEObject. Also, CLASSNAME is the ProgID!
Set PPTShp = PPTSld.Shapes.AddOLEObject(Left:=100, Top:=100, Width:=150, Height:=50, ClassName:="Forms.ComboBox.1")
'Print it out to make sure.
Debug.Print PPTShp.OLEFormat.ProgID
End Sub
Give it a try and see if that maybe fixes the issue.
Developer --> More Controls --> register custom? - A.S.H