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

Co:Writer Universal Microsoft Edge Add-on

*The Chrome app for testing has to be pushed out by the Google admin in Kiosk mode from the Google admin console.