In order to subscribe to streams, you must register an account on the bloXroute website and use a paid tier. Streams are available to all paying users that access the BDN with a Gateway or Cloud-API.
Note that Cloud-API streaming functions the same as Gateway streaming. The only difference is that the Cloud-API streaming can be used without a Gateway (and thus without a node). These messages are sent as JSON-RPC notifications over a websocket connection. In case you are using Gateway feed you need to enable the Gateway websocket RPC. The websocket RPC is off by default. Start the Gateway with the following additional arguments:
--ws-host <IP address of client application>