First of all, please try clearing your browser's cache and trying again.
This will ensure that your browser fetches and uses the most recent versions of the WP-Juicebox files from your web server.
If this does not help, then please try temporarily disabling all plugins other than WP-Juicebox to see if this helps. It it does, then re-enable your plugins one by one (checking the WP-Juicebox functionality after each one) until you find the source of the conflict.
If you use the BulletProof Security plugin, then this may be the cause of your problem and you will need to add a skip/bypass rule to the .htaccess file in your root directory as follows:
# Juicebox skip/bypass rule
RewriteCond %{REQUEST_URI} ^/wp-content/plugins/wp-juicebox/ [NC]
RewriteRule . - [S=13]
If you do not use the BulletProof Security plugin, then please check your .htaccess file for an entry such as:
<Files *.php>
deny from all
</Files>
If you find such an entry, either comment it out or remove it and WP-Juicebox should work as expected.