If you purchased avast Pro, you should have been sent an email with your registration key, presumably you saved this ?
A sale where, e.g. Where did you purchase this copy of avast Pro ?
Registering on-line to get your key sent to you is only applicable to the free Home version (no good for a pro version. However, you would still have been send a home license, if you didn't receive that your ISP may have an anti-spam filter at work, not that does you any good as it won't work.
You can contact sales (at) avast (dot) com giving the same information, but if you didn't purchase from a legitimate source (and there are fraudulent sites and sellers on auction sites) then even they may not be able to help.
After a reformat the demo key in the avast Pro trial should be good for 60 days giving you enough time to resolve this.