Note: Melodyne Essential is only included in the Studio One Pro version!
In your account go to "My Products / Software" and choose "Studio One Pro Details button".
You will now see a long list of downloads for Studio One Pro. Scroll down and after the "Extra Downloads" and before "Product Resources" you will see a download item for "Third Party Resources". Here you should find the Melodyne Essential Serial Key.
Above is a download item "Extra Downloads". Choose the column "User Manuals" and than select "Studio One - Quick Start Guide". In this manual you will find the installation steps for Melodyne Essential (chapter 1.7).
So I hope this is helpful, Gottfried