First, you will need to connect the domain you registered to the hosting package you purchased.

Start by clicking on My Account in the top right corner, then Hosting Manager.

Your Hosting Manager holds all your hosting packages. To add your domain to your Managed Wordpress package, please click Add Domain.

Enter the domain name on which you'd like to build a website.

If your domain is registered with Rebel and in your account, this will update the DNS to point to your new hosting package. Please note, the DNS will take a few hours to propagate, so this is a good time to get familiar with the Plesk Control Panel before opening Wordpress.