If it is not anymore, it was the case that there were separate 32/64bit Activation files.
 
I experienced the same as the OP concerning the Cakewalk MP3 Encoder. I contacted Cakewalk with the problem, they told me there were separate Activation files, they sent me a link to the 64Bit Activation file, I used that one and it solved the problem. That was during SONAR 8 or 8.5, and I saved both activation files, so I didn't have the problem when X1 was released. I just used the saved Activation files.
 
The matter may have changed, so that there's one Activation file for both the SONAR 32/64bit Cakewalk MP3 Encoder, but in the past there were separate files for each.
 
Additionally, this info has been helpful for other users who have run into this problem. The MP3 Encoder was working when using the 32bit version of SONAR, but not working when using the 64bit version of SONAR. They contacted Cakewalk, told them they were using SONAR x64, and Cake sent them a link to the 64bit Activation file.  Their problem was solved.
 
Go figure. :)

That would be great news indeed, if accurate.  Let's hope it's that simple.
 
SO, a tech support call seems to be the quickest way to a happy place with encoding MP3 data.
 
Bob Bone
 

I just installed a few days ago.  Downloaded the file from Cakewalk, ran it as admin.  Opened Sonar, export options included mp3 - no trouble.
 
However, I could not export an mp3 if the 64-bit box was checked.  That's a known issue apparently, folks here told me about it. 

olemon
 
However, I could not export an mp3 if the 64-bit box was checked.  That's a known issue apparently, folks here told me about it. 

 
That's a piece of good info.
 
I was reading of some problems with X3 when the use "64bit double-precision engine" box is checked, such as Console Emulator ProChannel modules producing noise on mono tracks, but I hadn't read that there's a problem with MP3 encoding working when that option is enabled.
 
Man, the bugs that are popping up when having that option enabled are causing some really irritating problems!

64bit double-precision engine box is not checked and am still having the same issue.  I will contact tech support in the morning to get this resolved.  Thanks to everyone for responding.