Surprisingly, this is the first time Ive tried to bounce to mp3 and I cant... I have searched to see if anyone has this issue and cannot find any threads on this matter.
 

This may not directly answer your question, but...  What I do is open a final mix in a basic WAV editor (Sound Forge, Audition, Audacity (uh oh, freeware... ! ) , etc.).  In there, I apply my final "tops and tails" trimming, then save it as a .wav file and then save as an .mp3 file.
 
-Dave

You can EXPORT as an Mp3. Bouncing is internal. Export is found in the FILE menu, make sure to "select none " first 
But myself I also do as Dave and only export as Wave and then "master" in a wave editor. 
I use Gold Wave for MP3 convertion as it does them as Batches which is what I normally need for a whole album. 
It's free to try forever but only $50 to purchase  and comes with a lifetime update ( yes really?) 
http://www.goldwave.ca/release.php
 
 

Works for me just as it did in Sonar Platinum.

its sorted thanks all...