Having the same problem registering Melodyne for Sonar Platinum as I did for X3.  I just gave up on it for X3 and never used it, which totally sucked.
 
1.  I insert Melodyne as an FX, and a window pops up asking for my serial number.  I enter the number and click "Continue Registration."
2.  A window asks me to log in or create a myCelemony account.  I don't have one, so I click on "create myCelemony account."
3.  A window pops up with my serial number saying that this number has already been registered!  And that I should enter my email address and password of my myCelemony account.  BUT I DON'T HAVE ONE YET!
4.  I try my cakewalk e-mail and password, but it's invalid.  WTFF?
5.  I try requesting a new password in case somehow I did already register while stoned.  I enter my e-mail, and it says no customer is associated with this e-mail!  WTMFF?
 
Any help would be appreciated.  How did you guys do it?
 
 

No point putting this here, Contact their customer support. They are very responsive friendly and most excellent for me. Twice now I forgot to unregistered before upgrading or changing my OS drive. They fixed this within a day both times. 
As far as creating an account it was same process as any registration. You must of hurried through it and somehow entered something wrong. 
 
And most of us are not downloading or using the version that comes with 2015. My X3 version is working fine still. I'll keep the new version on hold as it gives me 2 extra installs. 
 

Ok, it might be my fault, but I don't think so.  The registration process shouldn't be so easy to screw up.  I never gave them an e-mail or password.
 
Just wondered if anyone else had a similar problem.

But you just said you had tried with X3. You probably got half way though it that time so are in the system somehow. Just contact them and include your serial number from X3. 

I didn't change mine either, since I already upgraded to Editor 2 and didn't want to screw that up, with Essential.
No issues here.
Michael

I too kept Editor, I had upgraded with X3.
 
The registration for Melodyne comes through Celemony, not Cakewalk.  Hopefully they can straighten it out, as suggested by others.

Just got a reply from Celemony.  Someone else has registered my serial number!!!  Hacker!!!!

Have you changed your email with Cakewalk? Though I suppose you'd have had to change it with Celemony.
Good luck. Have you kept your emails with them? Buy it through the CW store?
 
You can straighten this out.
Michael

I'm sure the damage will be limited.  I did everything legitimately and purchased Sonar directly from Cakewalk. 
I'd like to give that hacker a knuckle sandwich.  Hackers are criminals, just like shoplifters.
It's early Sunday morning in Germany, so I don't expect a resolution right away.
 

'

No doubt you will be good as gold, next week. Good for you.
That's the attitude to get there, too.
Michael