Having problem on exporting x1 songs to mp3.  I turn up Master bus to 6.0 and input vol on Master bus to 6.0 and mp3 volume is still too low.  Please help.  Thanks

Check the mains level.
Are wave exports OK?
Are the mp3s being tested by re-importing into SONAR?
If all else fails there is http://mp3gain.sourceforge.net/

To me you are missing an important step in creating a proper release when exporting directly to MP3.
 Its called mastering. 
I recommend you export the song as a wave file and don't fuss with the loudness at this point,,, just get a good balanced mix. There are steps you should always take for best results:
Mixing stage, 
Mastering stage 
Then duplication and MP3 convertion. 
 
When your happy with the mix export it ,  then master the song so it's properly optimized in RMS average levels and eq etc,
then you replicate the songs= convert to Mp3, burn a CD etc.  
Combining mixing, mastering and replication/ conversion in one step is asking for trouble. 
 

Thanks for the feedback.  The main out is up to max volume as well.  Not ready for mastering.  Working with collaborators to improve lyrics and melody and easiest way is to send mp3 via email. In the past when I raised the master level I achieved a satisfactory level of the mp3.  Will export to .wav if I have too.

The problem is likely to be in the output that you are sending to be encoded to MP3.
 
Look at your project for some reason that the output level is being reduced. Is the master fader not at 0? If you are using the master fader to control listening volume, this could reduce the output level.

I would also check the mixer in Windows to be sure that nothing has been turned down there.

Master level is now at 6.0   Will check the windows mixer.  Thanks guys.

Hmm, the windows mixer should not be involved in ASIO devices,,,Your Sonar faders are in control of all audio. 
There should also be the interface bus in console view.  Double clicking any Sonar faders re sets them to unity and that's where a master buss belongs. 
If your meters are still showing low then you need to turn all your tracks up to get the master buss meters closer to "0" . Your meter is your guide. 
If the master buss meter is showing close to unity then your export will be at that level, and this should be regardless of format. But this can often result in a song that has a Average RMS level of -22Db. Modern songs are now around -10 Db. I go for -12Db. 
Peak level and average RMS level are 2 different things. 
If this is the case your next move could be to put the LP64 multi compressor or Boost 11 in the master effects bin and push it up a little.  This would be considered mixing and Mastering in one pass but for your needs is fine. I was originally concerned that you where sending out a finished product, not a demo. 
 
Another check is to mute your master buss and make sure the project goes silent. Often track outputs can get directed to your interface and are bypassing your master. 

sock monkey
Hmm, the windows mixer should not be involved in ASIO devices,,,Your Sonar faders are in control of all audio. 
There should also be the interface bus in console view.  Double clicking any Sonar faders re sets them to unity and that's where a master buss belongs. 
If your meters are still showing low then you need to turn all your tracks up to get the master buss meters closer to "0" . Your meter is your guide. 
If the master buss meter is showing close to unity then your export will be at that level, and this should be regardless of format. But this can often result in a song that has a Average RMS level of -22Db. Modern songs are now around -10 Db. I go for -12Db. 
Peak level and average RMS level are 2 different things. 
If this is the case your next move could be to put the LP64 multi compressor or Boost 11 in the master effects bin and push it up a little.  This would be considered mixing and Mastering in one pass but for your needs is fine. I was originally concerned that you where sending out a finished product, not a demo. 
 
Another check is to mute your master buss and make sure the project goes silent. Often track outputs can get directed to your interface and are bypassing your master. 

 If your asio device is set to default  in windows it will certainly have an effect.