I encountered this annoying issue once I upgraded to WordPress 3.5. Here’s how to fix the pesky problem instead of pulling all your hair out trying to figure it all out only to spend hours trying to fix something you can easily do in a few minutes!
Now onto the main issue at hand…
WordPress Fix – Add Media Button Not Working
If you find that after you upgraded your WordPress that you are unable to add any photos or media for that matter to your WordPress posts, all you have to do in order to get your add media button working consists of the following:
1. Simply login to your WordPress backend either though FTP or your Hosting company’s c-Panel.
2. Open your ”wp-config.php” file in the main folder associated with your website
3. Search for the ”require_once(ABSPATH . 'wp-settings.php');'' within the ”wp-config.php” file
4. Insert the following code
define('CONCATENATE_SCRIPTS', false );
* Make sure you insert this code BEFORE and INFRONT of the line that reads ‘require_once(ABSPATH . 'wp-settings.php');''
This is how I fixed my add media button issue in WordPress and I hope it works like a charm for you as well ( refer to the above actual quick fix photo if you need to have a visual) =)