This may be an issue with the default NetworkManager configuration. By default, NetworkManager won’t touch the default wired ethernet connection because it’s listed in /etc/network/interfaces. As a consequence, it thinks the system is “offline”. I think this is greying out the “add” buttons. (It also breaks Evolution.)
Performing the config changes listed in https://wiki.debian.org/NetworkManager causes both of these problems to be fixed. This should probably be the default configuration.
