When hook_field_load and hook_field_formatter_view are called? I have implemented the hook_field_load() and hook_field_formatter_view() function to replace some characters in a specific field's value. But they don't seem to be called at all.
I wrote some command like exit() and print_r($field) in the hooks but nothing happens.

When this hooks are called? (and is this the correct way to do alter field values for view?)
hook_field_load() and hook_field_formatter_view() aren't hooks really. They rather are callbacks which only can be used from a module implementing the field in question by itself. The documentation at api.drupal.org for both hooks is not very clear in that by stating nothing or just a vague: 


  Define custom load behavior for this module's field types.


When grepping through the Drupal 7 code you never see this so called hooks invoked by module_invoke_all() which is a clear sign for that functions are not handled like hooks.

hook_field_attach_load() may be used instead of hook_field_load() which is a real hook.

See also the discussion at http://drupal.org/node/702586 for that matter.