Channel security functionality is only available in Pro subscriptions.
By default, your channels are accessible by anyone who knows the exact playback addresses. If you have a Pro subscription, you can whitelist a number of IP addresses or IP address ranges. By doing so, you make sure that only playback devices with an IP address that matches the whitelist can access your channels.
To define your whitelist go to the company settings page and type an IP address or IP address range (using wildcards, e.g. 82.89.*.*) in the "Allowed IP addresses" box. Each allowed IP address or IP address range needs to start on a new line.
If you don't want to impose a playback restriction based on IP addresses, but give everyone with the right channel address the ability to play that channel, then leave the box empty.