Until now, Skype did not allow voice calls on Chrome OS. Now it does. It does not allow video calls yet, but voice is at last there and works as it should.
You can use the web version of Skype at https://www.skype.com which will take you to the login page, or you can add the Skype extension to Chrome which gives (almost) instant access from the browser.
Or, of course, you can do both.