Cakewalk has a licensing agreement with the encoder that comes with Sonar, and it runs out of free encoding after I believe 30 days.
Scook is correct, you can either buy the license or you can use one of many many free ones available, and configure one of those to work within Sonar.
Your other choice is to export as wav from Sonar and then convert to MP3 outside of Sonar, again with one of a zillion converter programs running around for free.
I believe the same encoder that comes with Sonar, the one you could buy, is available as a free one outside of Sonar, that you could then configure to work within Sonar.
I just said screw it and bought the encoder license from Cakewalk, and have never looked back. It was something like $20 at the time.
That same encoder is noW available for $9.99 which is ridiculously inexpensive. I just looked at it in the Cakewalk store.
Here is that link, if you are interested:
http://www.store.cakewalk.com/b2cus/productinfo.aspx?productid=10-cwfh1.05-10e Bob Bone