I cannot add elements to my video

You can't add pictures or videos because you cannot click on the button "Upload from your computer"? Or you cannot upload any music? 

First thing that we always recommend: use the browser Google Chrome instead of Internet Explorer or others (Firefox, Safari, Opera).
This browser works best with our tool and using it will probably help you solve this issue.
Check that you have the latest version in your Browser "About Chrome".

We also recommend to have the latest version of Flash player installed:   http://get.adobe.com/flashplayer/