To use the Chromecasts with the iPad you have to find an app that works with it, click on the cast button (shown below), and choose which table you want it to cast to.
You can find a list of apps that work with the Chromecast here.
On the Chromebook or desktop you just have to click on the menu button (shown below) in Chrome, click cast, and choose which table you want to cast to. If you don't see the cast option then you need to update your Chrome browser or Chromebook. Click here for a tutorial on how to update Chrome.
After you click Cast, choose the device you want to cast to and make sure the TV is switched to HDMI 2.
To cast to all TVs, choose ElemTechroomBigScreen and switch the small TVs to HDMI 1.