Hi Harry,
you need to download the zip file to your PC.
From the zip file copy following jar files to the B4J additional library folder: pi4j-core, pi4j-device, pi4j-gpio-extension.
Ensure you also have the jPi4J.jar and xml in your B4J additional library folder.
To run:
On the RPi start the B4J bridge: $sudo java -jar b4j-bridge.jar. Note the IP address.
On the PC start the B4J IDE, connect to B4J Bridge using the noted IP address. In the IDE bottom left, connected is displayed.
Run the app by pressing F5 ... and the LED should flash every 5 seconds.
Good Luck,
Rob