Was running in 32-bit X3 before.  Is it a 64-bit compatibility issue?  I get the message "The MP3 Encoder must be unlocked before use."

It's working fine for me. Reach out to support - maybe they have an updated version for you?

Contact tech support for the 64bit unlock program. There should be no charge because you already have the 32bit version. The 64bit version of SONAR does not see the 32bit unlock code.

I contacted support yesterday morning through the website.  They responded about an hour later asking what message I was getting.  I responded immediately and have heard nothing since.  I guess I need to be patient.
 
I bought the encoder years ago and thought maybe I needed to buy an "updated" one, so I purchased again yesterday.  I think it's the same product that was updated in 2004.

I called support when I moved to 64bit. In less than 5 minutes there was a link to the 64bit encoder in my email inbox. Purchasing the mp3 encoder is a one-time event. The license is good for all Cakewalk products forever.

Just contacted support last night for the very same thing.
They replied very promptly and also included a download link for the encoder in case the problem of it not appearing on list of purchased products persisted.  Tested and all works a treat.