I have the Scarlett 6i6 and the Scarlett 18i20 is just like the 6i6 only with more inputs and outputs. The Scarlett MixControl software is not the most user friendly software and Focusrite could definitely improve on this. Both the 6i6 and 18i20 use the Scarlett MixControl I'm going to try to explain how to do this and hopefully you won't get too confused. BTW I have a video on Youtube that may help you wrap your head around the Scarlett Mix Control Software. Granted I'm talking about the 6i6 in the video but the same applies to the 18i20:
http://www.youtube.com/watch?v=VDo7-fzzKtY
First off I would say setup Ableton Live first with the following:
On the Master Channel set the Master Out to outputs 1/2
Set the Cue Out to 7/8 which is your headphone jack 1
Ok now for the hard part. Open the Scarlett Mix Control and prepare to be confused:
By the way if you want to save your current mix preset before you make any changes go to File, Save to save the current Mix Preset. You can also always reset to the factory default by going to File, Save
More than likely your Routing Preset is probably set to Mix 1 (L) and Mix 1 (R). This will do for now.
Below Routing Preset look for Mix 1 (L) ->Line Output 7 and and Mix 1 (R) ->Line Output 8
Change this to: Mix 3(L) ->Line Output 7 and and Mix 3 (R) ->Line Output 8
Now still in the Scarlett Mix Control look for the Master Fader on the Mixer. It's on the far right and should be labeled Mix 1 at the bottom of the fader.
At the top of that fader you'll see Many... Click the left one that says Many and uncheck everything except Mon. 1
Click the right one that says Many and uncheck everything except Mon. 2
This sets everything from the Mix 1 tab to only go to the Main Output of the Scarlett 18i20.
Now on the other faders you should see DAW 1 / DAW 2. Make sure this fader is set to 0db.
For now until you understand the mixer set all the other fader volumes all the way down.
This will allow the Scarlett Mix Control to only hear the the Master Output 1/2 from Ableton Live
Ok, Now on the top of section of the Scarlett MixControl, Click the Mix 3 Tab.
Look for the Master Fader on the Mixer on the far right and should be labeled Mix 3 at the bottom of the fader.
At the top of that fader you'll see Many... Click the left one that says Many and uncheck everything except Anlg Out 7
Click the right one that says Many and uncheck everything except Anlg Out 8
This sets everything on the Mix 3 tab to go to the headphone output 7/8 (Headphone Jack 1)
Still on the Mix 3 Tab, set the DAW 1/DAW 2 fader to 0 db.
This is so you hear your main mix in the headphones
Set all the other faders volumes down so you can't hear them.
Look for a fader that says DAW 7/ DAW 8. If you don't have one we need to create on.
Click on any fader that says off and change the left to DAW 7 and right side to DAW 8
Set this fader volume to 0db
This allows you to hear the click track in the headphone mix (Mix 3 Tab) as well as the main output mix from Ableton (DAW 1/DAW 2).
If all goes well you should be now be able to hear your click and main mix in the headphones. Your main output should not hear the click.
I really recommend that you watch the video I did to help you further wrap your head around the confusing Scarlet Mix Control Software.
HTH