that dident solve anything as I always select the whole track drag till its gray and click on every track yet some how all 4 minutes and 23 seconds of the track are mixed down but nothing comes out the first 4 bars that starts out the beat are of a piano midi riff that I dident record the parts that I actually played and recorded come in after the 4th bar and play in the mp3 fine when there supposed to but all the other midi files are missing out of it only what I played is mixing down

When you export, make sure that "Fast Bounce" is unticked, as it doesn't always work well with some softsynths.
 
Also, as Jonesey said, export as a WAV first. Convert the WAV to MP3 later as a separate stage.