The Website Builder, provided by Weeblycloud, is an add-on that can be purchased via your dashboard. 

Head to your Radio.co Dashboard > Marketplace > Website Builder and choose a plan.
Both include a free domain. Pricing and features are available on the page in your dashboard.
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Once payment is complete, you can continue setup. If you have your own domain already click 'Existing Domain'. If you need to register your free domain, click 'Register Domain' to check availability. 

Once setup you can access your website builder editor at any time by going to Marketplace > Website Builder, and clicking the edit button.