I'm guessing, but which browser are you using to restore your license? It sounds like you restore a license but its not being picked up by MC. Some browsers or addons can interfere and rename the file to something that is not understood by MC.
Try a regular Internet Explorer and check the file when you restore. The file should end with .MJR.
Also if its not too long ago (14 days) a license file should have been mailed to you. You can try and save this file from the email attachment.
I did the most recent install of MC18.0.158 on a machine running Firefox and actually received two emails (both the same) excerpted as follows:
Temporary Install Key for JRiver, Inc. Media Center 18
Date: Tue, Apr 23, 2013 11:21 am
[Attachment]
Hopefully, since Registration Information indicates that MC18.0.158 is REGISTERED on both machines, I will not burn thru remaining restores.
The necessary procedures as documented for registering, restoring, etc., are quite confusing as evidenced by fact that I am down to 4 restores without even realizing how I did it. Hopefully, the new (and improved) procedure for a restore as indicated by Matt will resolve issue for those like myself who just want MC to run on a couple of machines for personal use without messages indicating that trial period is running out.
Thanks.