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 for use 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