register it? If you're referring to registering mIRC go to this page
Register If you've loaded a script that requires you to register it, personally, I'd be getting rid of it. mIRC scripting code is open and shouldn't require any kind of registration.
If you don't have the script loaded, type
/load -rs <script name>
exchanging the actual name of the script for <script name> and it will usually end in .ini or .mrc