You have CB Activity installed, but it's not published in CB > Plugin Management. If it's not published it won't load on frontend so the module is going to tell you it doesn't exist. I've published it for you. CB Activity works fine, but you're going to have the same issue with various other plugins that you have not published. I highly recommend reading the CB Primer Book, which you can download below, to get started with CB as we cover plugin installation and basic usage in it.
www.joomlapolis.com/downloads/documents
We also have several video tutorials below that cover various plugin topics and basic CB usage.
www.youtube.com/user/Joomlapolis/playlists