Can you better describe what you mean by it's not working? Does it give a validation error? Does the button just do nothing? What browser are you using? What Joomla version are you using? Please be sure both CB and CB Activity are also up to date. Have retested and appears to be working fine in my tests.
The below is from our demo site.
Within System > Manage > Plugins edit "Content - CB Activity" and under Comments ensure "Stream" is set to "Articles". Sometimes that gets messed up if you migrated from an older install of CB Activity.