Dj Beasey if you have a tt57 or 68 this is what you need to do to achieve what you are looking for all you need is another sound card as Sam suggested then follow these steps and you good to go
1st the hardware routing. it doesn't matter what soundcard you use..
1. 1 set of cables in your sound card out routed to the flex fx in.
2. 1 set of cables in your sound card in routed to the flex fx out.
3. your speakers HAVE TO BE attached to your mixer for this to work. (master or aux)
4. arm the flex fx button: if you want to record the left deck, push the left button in. if you want to record the right deck, push the right button in.
now for the computer stuff..
1. open ableton
2.open serato
3. open ableton preferences
4. under driver type, select your soundcard (not the serato driver) then select the proper in/out settings pertaining to your sound card.
5. load empty audio track in ableton
6. (arm record) in the audio track you've loaded there will be a little rectangle with a circle in it. click that with your mouse. it will turn red.
7. load your ableton set on the serato deck you DO NOT want to loop. (see the bridge pdf instructions if your confused)
8. load any file you want on the serato deck you DO want to loop.
9. push play on both decks. you should now see your signal level in ableton regerstering.
10. push rec. if you don't know how to do record in ableton you need to get familiar. there are many settings and tools for looping in ableton live. what settings work for you is what works best.
after you record into your track in ableton the audio will be bounced on to the turntable with the 'ableton set' playing on it.