Hi!
I have a 13-inch early-2009 Macbook, running OS X Yosemite. I have had this problem ever since I upgraded my OS to OS X Mavericks months ago. I also remember encountering this problem when I first upgraded my OS to Mountain Lion, and I had to search up possible solutions on the Internet, particularly those threads where Mr. Ralph Johns had commented on. I was able to resolve this problem back then (thanks to Mr. Johns' advice). When this happened again after upgrading to OS X Mavericks, I tried going through the same options to get the iMessage working (which involved changing files' names and contents, editing some text files, re-authorizing my computer in iTunes, etc.), but to no avail. I upgraded to OS X Yosemite last night (even did a clean install) for hopes of resolving this. Still, nothing happened. I kept getting the same error message.
I then read up somewhere that the Apple Support won't charge you for iMessage activation, so I finally gave in to what others called the "last resort". Eventually, I called up Apple Support, and I was asked about my validation code and my Apple ID. Afterwards, they asked me if I still remember the answers to the security questions that I have set up with my Apple ID. You have to give at least 2 correct answers (there are 3 questions). Then the tech support person will activate your iMessage on their servers for less than a minute. He/She will then ask you to sign in to your account once more to see if it's already working.
Mine worked immediately after the first attempt.
The tech support guy was very accommodating just like the other tech support people I have talked to in the past. There was never a mention of charging me for their service, and they didn't have to ask for my Mac's serial number at all. Moreover, there wasn't any mention of my product warranty or whatsoever. iMessage activation should be free. If they want to charge you for this service, you may want to talk to their superiors. They would know better.