If you have paid for mIRC, the registration number should work as it is a one time fee and free upgrades after purchase. I would try:
1) make sure you are entering the complete registration number. (as it may be possible you left a number out)
2) completely uninstall and re install mIRC from a fresh copy of mIRC downloaded from this website.
3) Double check the registration number by using the Lost Registration
form (even if you didnt lose it, it can be e-mailed again)
4) Make sure you unblock mIRC from your firewall settings
NOTE: These are just suggestions. Try any of these to see if they help any. If the problem persists, please post again and we will try to help you further.