This just means that the IRCd for the server you're on doesn't have an 'o-line' (a line in the ircd.conf that identifies you as an oper) for your hostname. This is nothing to do with mIRC, it's a server issue, and as Mtec said, you'll need to speak to a server administrator to get your IP/hostname added to the server.
Server administrators are the only ones with the ability to do this (sometimes Assistance Admins too). Check the /motd of the server to see who the server admin is for the server you're using.
Regards,
