You should use a PC card with SPDIF in - either RCA or optical. Then it's quite easy, as you can do the mixing on your EX and 'record' on your PC.
I always go a different way and do a mix to a CDR (got an audio cd recorder) and rip the CD afterwards. Why? Well you can listen to that cd before mastering (which I - alas - do myself...).
aXel