It looks like you're new here. If you want to get involved, click one of these buttons!
Someone asked how to do this today, so this is basically a summary of my step-by-step instructions (with pictures!) of my instructions how to do this on my blog:
Again, this assumes you have a humidifier that is always powered (like with a 110V wall plug) and merely actuates the solenoid to activate the humidifier when it's called for by the Ecobee.
1) Use a short piece of wire to connect RH to ACC1r on the Ecobee EI.
2) Use a long piece of wire to connect ACC1 on the EI to either of the solenoid leads on your humidifier (I promise - it doesn't matter which one).
3) Connect the COM terminal on your furnace to the other solenoid lead on your humidifier.
4) (optional) On your Ecobee, go to Installation Settings, and turn the "Humidify only while Heating" to "No" if you want that option.
And that's it! It should take you 5 minutes to wire it all up. Again, hit my blog link above for close-up pictures of the wiring.