How can I add a progress bar to Managed File upload?
managed_file fapi elements are already progress bar capable by default.

managed_file


  Description: Provides a complete ajax/progress aware widget for
  uploading a file and saving it to the {file_managed} table.
  
  By default a simple upload button is provided for choosing the file to
  upload. Once a file has been chosen it can be explicitly uploaded via
  the "Upload" button. The upload is via AJAX and a progress meter is
  displayed.
  ....
  
  Non-standard form element properties:
  
  #progress_indicator: options are 'none', 'bar', and 'throbber', default is 'throbber'.
  #progress_message: (string) Progress message to display along with progress meter while a file is being uploaded. Defaults to NULL.


It may be possible that your server doesn't support file progress, in which case you should check and verify if the server settings are correct.

More info: 


https://drupal.org/node/793262
https://drupal.org/node/793264
https://drupal.org/node/654732