I owend one of those units a couple years ago, so im just going from memory. I dont think there is anyway to get it on a computer without burning the engineered stereo tracks to a Seperate CD burner.
1) mix your tracks.
2) burn it to a cd using the RCA stereo outs to a external CD burner.
3) put the cd in your computer.
4) convert to whatever format will sound best, mp3, wma, whatever.
5) email to you mastering guy.
I would think you may just want to mail the CD due to the poor quality of emailing music, unless you have a really great computer set-up.
good luck, i hope it works out for you