To syndicate listings to Radius you will want to ensure that you have a Radius account and that you have signed in to it at least once.
You can create a free Radius account by visiting https://www.radiusplus.com/signup. For help signing up as a broker checkout this article.
Now that you have a Radius account you will want to head over to settings at Buildout.com.
Within the Buildout settings there is a syndication tab where you will find Radius+ under Available Channels.
This is where you will enter your Radius details to connect the accounts.
Pro Tip: To make sure your listings stand out in Radius you will want to have image syndication enabled and ensure your property profile is filled out in detail.