there are 3 tempo modes on the 909, one to act as a master, two to act as slaves.

when in track play mode, hold down shift and hit the tempo mode button (last one on the right, to the left of the accent button), you should see the display change through "135" (master tempo mode), "__0" and "_0_" (slave modes).

can't remember of the top of me head which slave mode is which, but it could be that you've been in the wrong tempo mode is all that's wrong.

If I was using it with ableton, I'd use it as the master tempo (in the tempo mode that displays bpm) and sync everything from it. much easier and you can utilise the 2 midi outs if necessary.

that's how we used to get our macs in sync back when ableton wouldn't send out a midi clock (v1).