Hi Keni:
If you paid for the Cakewalk MP3 Encoder activation, check your Cakewalk store account for the link to download the Activation file, if you're using SONAR X64, you may have to contact Cakewalk for the x64 Activation download link. The one in the store account used to be for the 32bit version of SONAR. I just keep the Activation file for both 32bit and 64bit, so I never had to download it again.
When you use the Cakewalk MP3 Encoder in Platinum (latest version) it uses Lame 3.99. Sometime during the SONAR 2015 release, Cakewalk, finally :), updated the Encoder to use Lame 3.99.