If you use a regular (non-ACF) field with the code I gave you, does it work?
That code works for me on my test system, so I think the problem is with getting the ACF field value using get_field().
Try to test this without MapPress. Add your code with get_field() into the template file for single posts (usually in the theme directory ‘single.php’). Do you see a value that way?
If not, then you’ll need to ask the ACF folks why the value isn’t coming through.