That error is specific to the server you use and nothing to do with mIRC. It means you need to register a nickname or username for you to be able to join the channel. For information on registering (which also has nothing to do with mIRC), ask in your network's #Help channel, or check the network's website for online documentation.
The "MOTD file is missing" is nothing for you to worry about, and again, is specific to the server you're using. MOTD means Message Of The Day, and it displays when a client connects to the server. All this means is the file for the MOTD is missing.
Regards,
