Just click on the Create with this... button on any video on the WholeWorldBand website. You will be taken to the Arrange page where you can pick which tracks you would like in your video. Once you have chosen these, simply click on the Continue (Launch App) button, which will launch the Youdio Music Video Creator app.
If this is the first time using Youdio, you will be prompted to download and install the Youdio Music Video Creator. You only have to do this once - after that it will work automatically in future.