Not having a registration number will not keep you from being able to use IRC-- you should be able to freely connect to any server you wish even while not registered, just close the nag screen.
As far as your registration #, rest assured, it is likely on its way-- email is theoretically instantaneous, but not always.. check your junk box to see if it maybe got accidentally flagged as spam, and if not, wait-- in rare cases (keyword: rare, this probably won't happen) it can take a day or so, just use mIRC unregistered until then.
