Nickname registration is related to the network you're registering on - some networks don't have an option to register your nickname, some have different Services for registering a nickname and therefore, have different commands. If the above doesn't help, try out the #help channel on that network (or other official network help channel or the network website.

Also, as an FYI, you do not register your nickname with mIRC you register it with the IRC network (and its Services).