Thanks for purchasing the conversion license.
You mention in the title that you were using 5.0.0rc1. It's better you use 5.0.1 that you can download from
https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/versions/But that's probably not the issue here. If you installed ConvertHelper at the time you installed 5.0.0rc1, chances are that you are running ConvertHelper 3.1. This version, on some 32 bits systems, causes the exact error you describe, because of a runtime DLL you may lack in your OS. Please, go back to
http://www.downloadhelper.net/install-converter3.php and re-download/install ConvertHelper, which should then appear in version 3.1.1.
Let us know how it goes.
Of course, if you cannot use our product, we will refund your purchase.