To subscribe to Radius+ Insider simply navigate to your profile in the top right. Towards the middle of the page you will find a link to subscribe. If you have a credit card on file you simply need to confirm the subscription otherwise there will be a field to enter your credit card details. 

To manage your subscription, cancel it or find invoices simply navigate to the billing section of your profile. 

For details on various Insider features check out the following articles: