You should either be able to log out of the old core on the mac (you would need to start that core up again just to do this) or de-authorize the old core on the Nucleus. It should give you the option to do that when setting up the Nucleus. Can you post a screen shot of what you are seeing on teh Nucleus?
Can to describe what you are trying to do? What device are you on attempting this on? Did the device previously connect to the old core? Are you sure you are trying to connect to the correct core?
What build number is the remote you are running? It looks to me like the Nucleus is running an old build. Is it a brand new unit?
If you can get to the WebUI I would try re-installing the OS.
The first thing to do when powering up a new Nucleus is to update the firmware. @bearFNF has posted the link to the Updating Nucleus article. Please follow the steps given in the article.