If you have ever purchased an mp3 license from Cakewalk, contact support and they will provide the activation program free of charge. However, if you have not purchased a Cakewalk product which included an mp3 license such as ProAudio9 or have not previously purchased a license for the mp3 encoder and want to use the integrated encoder you will need to purchase the activation program from the Cakewalk store. Here is a post describing which product to purchase
http://forum.cakewalk.com/SOLVED-MP3-export-from-a-track-m3272801.aspx#3273106. SONAR also provides a method to add any command line encoder program into SONAR using the External Encoder Configuration Utility on the SONAR Utilities menu.