I have tried to download the MP3 encoder from Sonar several times over the past week or two, and they never send the download to unlock it, even though I enter my email address. I also updated my Sonar X3 Producer version to the X3e build, that was suppose to include the unlocked MP3 encoder. Still says the encoder needs to be unlocked.

Downloaded the Command Center and it doesn't show any of my registered products, though when I log into the web site they are all there.
???

Just install the new Bandlab version of Cakewalk which includes the encoder unlocked. MP3 encoder has been unlocked for a long time now. No need for a download it. 
  You need to log on to CCC with your same log on you use here if that's your Cakewalk account. CCC doesn't work with X3. 
You are aware that Sonar died back in November and there is no more official support so your E mails are going no where. Please read the stickies at the top/ 

X3 and older products are not available through the Command Center.
X3e still had mp3 locked. Even newer products had mp3 locked until April 2017. Starting in April 2017 Cakewalk begain shipping their DAWs with mp3 unlocked. They also published the mp3 unlock request page. Gibson shutdown Cakewalk in Nov 2017.
 
The old Cakewalk page offering the mp3 unlock program may not be working anymore. You could ask BandLab support if they will send you a link to the program or you can create a BandLab account with the same email address as your Cakewalk account, download and install Cakewalk by BandLab. It would be an upgrade to X3 and includes the mp3 encoder. CbB will make use of all the plug-ins bundled with X3 so leave it installed if you elect to upgrade to CbB.
 

I installed the CbB. Was a little wary of free software when it was announced and wanted to see how that panned out before installing. Really hated that crap Gibson did (offering lifetime updates for sale, then dropping the product). They did a similar purchase/discontinue with Gigastudio and GVI that I also had purchased. No fan of Gibson, for sure.
I'm more of a performer than recording artist, so I haven't used Sonar on a regular basis in the last couple years, but had some tracks that I wanted exported to MP3 and that is what restarted my journey to CbB lately.
Thanks for the replies.