On each candidate switch from the command line interface (I'm not sure if you can do it from the web interface), you need to issue the command (from config mode) "cluster run" which enables clustering on that switch. I have also run into issues where a cluster commander can see candidate switches but can't add them unless that password is already the same. If it is not the same normally the web interface will start a pop-up window and ask for the password, but I have seen it where it does not and I have narrowed it down to the password being different on the candidate.