You cannot use Beaver Builder with your WordPress.org account. An account with WordPress.org lets you participate here but it doesn’t give you a website.
The account for hosting that you have with domains.co.za is where you’ll want to install Beaver Builder.
Whatever domain you have linked to your account there, you should be able to access the WordPress admin of your site by going to
yourdomain.com/wp-admin/
Once you login there, you go to Plugins > Add New > Upload File and upload the zip of Beaver Builder that you purchased.
Then you can activate it and get started.
Your hosting provider may have another way for you to access the WP admin as well but you’d need to reach out to them about that.
Since you mention that you purchased Beaver Builder, if you need further help, please login to your account with us on our website and go to https://www.wpbeaverbuilder.com/beaver-builder-support/ to submit a ticket.
Per forum guidelines, we are not able to help you here when you are using a premium version of our product.