Klanghelm plugins don't need a serial number.
From your acount just download your plugin and unzip the folder.
You have two download options:
1) Installer
The installer will copy your plugins into the proper plugin folder location.
2) Zip file
Copy your plugins manually onto the proper plugin location.
Some valid plugin folders are:
X:\Program Files\VSTPlugins
X:\Program Files\Steinberg\VSTPlugins
X:\Program Files\Common Files\VST3
So, there is no rocket science, I hope this is helpful, Gottfried