That knob on the left :)
I'm sure there's a way to do it using midi, but you're in complete control of the drum loops so why not just beat match like normal using the tempo knob? You should already be fairly aware of the BPM when you get there, no? I've done it in the past.
The real problem comes with dropping in. It's difficult to get the loop starting points to match up. It's essentially the same as only being able to beat match using the pitch slider and the On/Off button, but not being allowed to touch the vinyl, pin, or platter. With a little practice, you'll get used to the slight delay and you can drop it pretty close to where you want it to go, then just tweak it till you're matched and off you go. Be sure to keep one finger on that tempo knob, though. The 909 can't do 145.5 BPM, so it will slip if your record is sitting on a middle decimal.
That's the cheap route, anyway. If you want to get beat counters, then some sort of midi device that turns your 909 into a tempo slave of said counters, then good luck! That's the only way I could see it working, but I'm sure it's possible.Get to your local synth store and ask the nerd on call if he can help you out.