If that is in mIRC's options rather than the script, press Alt-O > Connect. Change your main nick there. If it's the script doing it, then look for some main settings dialog that has a nick in it and change it there. As mentioned, OmenScript and other scripts are supported better where they are made rather than here. Many of us don't use that script and don't know the way it is set up.
#Invision on irc.irchighway.net