It does not matter which way the listener firmware gets on the cube, whether USB or WiFi. Processing just runs the game code and sends the graphics to the listener running on the cube.
If you have Cubetube working alright with your cube then one way to get the listener on your cube would be to create a new "L3D" type demo on
cubetube.org and then paste in the
listener code and send it to your cube. L3D Invaders is pretty old relative to all of the changes that have been made to
cubetube.org and the cube software, so I'm not sure that will work smoothly (worth a try).
A slightly harder alternative is to install the
spark-cli tool and use that to load the code over USB or WiFi. After installing the tool you would run "spark setup" to get logged in and connected. Then you would download the
listener binary, connect your cube to your computer over USB, put it in listening mode (by holding the mode button until the LED blinks blue), and run "spark flash --usb
path/to/Listener.bin".
Space Invaders may get uploaded to
cubetube.org again (it was purged in the recent overhaul that the website got). But it's not planned at the moment.