I checked your script (without any changes), and it works well.
The marker is shown in Australia near to Newcastle.

i seem to be having a similar issue and i have double checked the arguments im parsing and they seem to be in the correct order but my 3D prefab is inactive at runtime and the scale is incorrect. The code is below

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ThreeDMarker : MonoBehaviour {
   
        public GameObject markerPrefab;

        private OnlineMapsMarker3D marker3D;
        double markerLongitude= 151.5496154;
        double markerLatitude = -32.9765848;

        private void Start()
        {
            // Get instance of OnlineMapsControlBase3D (Texture or Tileset)
            OnlineMapsControlBase3D control = OnlineMapsControlBase3D.instance;

            if (control == null)
            {
                Debug.LogError("You must use the 3D control (Texture or Tileset).");
                return;
            }
        // Create 3D marker

        marker3D = control.AddMarker3D(markerLongitude, markerLatitude, markerPrefab);
           

        }
    }

any help or tips would be greatly appreciated!

Regards

You confused the latitude and longitude in coordinates of the marker.
X - longitude, Y - latitude.

working movie -> https://youtu.be/kdAvAn7hvpM   (youtube)