First make sure you installed ConvertHelper 3.1.1 (assuming you are non Windows) from
http://www.downloadhelper.net/install-converter3.phpNormally if you had a former license setup it should be updated or if you just purchased a license from a Firefox browser where VDH is installed, it automatically validate the license.
If this is not the case, open VDH settings > "More" > "Conversion", click on "Register an existing license", copy / paste (CTRL+V) your license key, click "Register license". You should now see your license data.