There should be an icon pointing downward next to your Display Name on the top-right of the screen - Click that and click on "My Library" It'll take you to the download options.
Alternatively, you can check the email you used to purchase this from and search for 'radiant'. If you (somehow) lost access to both the account you purchased this on and the email address, there isn't anything you can do - bar repurchasing it.
Hope that helps.