I use Ableton 4.1, though 5 is the same I think.

Go to Options/Preferences. Go to the Audio Tab. Click Output Config and enable the channels you want to use on your sound card. Click OK. On the very right most side of the screen towards the bottom half of the mixer section you'll see a little icon that looks like 4 little boxes stacked on top of each other in a circle. If you move your mouse over it it should read: Show/Hide Input/Output Section. Enable this and you'll see routing information pop up on top of your mixer. Select the relevant outputs under Audio To and you're done! If you need a screenshot let me know (but try first please!).