You can also zoom by using MKCoordinateRegion and setting its span latitude & longitude delta. Below is a quick reference and here is the iOS reference. It won't do anything fancy but should allow you to set zoom when it draws the map.
MKCoordinateRegion region;
region.center.latitude = {desired lat};
region.center.longitude = {desired lng};
region.span.latitudeDelta = 1;
region.span.longitudeDelta = 1;
mapView.region = region;
Edit 1:
MKCoordinateRegion region;
region.center.latitude = {desired lat};
region.center.longitude = {desired lng};
region.span.latitudeDelta = 1;
region.span.longitudeDelta = 1;
region = [mapView regionThatFits:region];
[mapView setRegion:region animated:TRUE];