Please note that we did not write the Juicebox module for Drupal ourselves and if you have any queries relating directly to the module, I would recommend that you post in the Drupal forum where the author of the module (who will be more familiar with both Drupal and the module itself) should be able to help you out further: https://www.drupal.org/forum
The Juicebox module for Drupal is an unofficial plugin but is well supported by its author on the Drupal forum.
Also, the link you provided points towards an alpha (pre-beta) version of the module so there may well be bugs in it.
If you find any, I'm sure the author of the module would be very grateful to know about them so that they can be tracked down and fixed.
Having said that, your problem sounds likely to be related to either the location of the Juicebox files on your web server or the permissions of the files (or parent folders).
Please double-check that you have uploaded the contents of the 'jbcore' folder to the correct location on your web server, step #3 in the instructions here: https://www.drupal.org/node/2000300
Copy the contents of the 'jbcore' folder (not the 'jbcore' folder itself) into the '/sites/all/libraries/juicebox/' folder on your web server.
Also, make sure that the permissions of the files and folders in the path are not too restrictive.
Default permissions of 755 for folders and 644 for files should be fine.
I realise that this may not directly solve your problem but I hope this points you in the right direction.