You may download the Co:Writer Extensions and Apps using the links below.
For Google's Chrome browser for use on Chromebooks, Windows, and Mac OS X
Co:Writer Universal Extension for Chrome
Co:Writer Universal Chrome App in Kiosk mode for testing on Chromebooks*
Co:Writer Universal App for Chrome
For iOS (iPads and iPhones)
Co:Writer Universal App and Keyboard for iOS
For Microsoft Edge Add-ons on PC and Mac OS X
*The Chrome app for testing has to be pushed out by the Google admin in Kiosk mode from the Google admin console.