The only reason I can see that the field group would not appear in the admin is if for some reason your field group setup is happening before acf is included and generally the only way that can happen is if acf is installed as part of your theme.
You can try adding something like this just before to see if this is the case
if(function_exists('register_field_group')){
die('function exists');
} else {
die('function does not exists');
}
either way your php will stop executing at this point and give you a message indicating if the acf function exists or not.
