A couple of issues that I see:
AGSPoint* myMarkerPoint = [AGSPoint pointWithX:lat y:lng spatialReference:mapView.spatialReference];
X is longitude, Y is Latitude
Then, what is the spatialReference of your map view? Is that the same SR as the lat long values you are getting? If your basemap is one of the ESRI maps, typically those are SR web mercator, and if your lat/long points are decimal degrees those are typically in SR WGS84. If you set the spatial reference of the point to WGS84 that should do it, but that's only based on the assumption of your error could be.