Currently there is no admin interface. You need to login via an admin account into the XMPP server using e.g. Pidgin or another XMPP client. An ad-hoc command will appear that can be used to send a message to all users.
Admin accounts are defined in init.properties (default is admin@domain). You need to create a personal key for it, you can use our certificate generator: https://github.com/kontalk/certgen (run it without arguments to see usage help)
More information on how to use a XMPP client with Kontalk here: https://github.com/kontalk/client-adapters