What part are you specifically getting stuck on? Knowing what your servers are running is helpful too. Make sure you're using the NetBIOS domain name in the first box, not the fully qualified domain name.
thank you for the heads-up... this might come in handy in the future since I will be integrating multiple organizations MX into their respective AD. I have 2 more question, hope that you can give clarification.
1. With integration guide on the Documentation, for me, I might need detailed instruction plus the full requirements as it is not stated.
2. With 2 MX on different sites, can I extend my AD thru meraki via Auto VPN?
1) As far as the requirements go... you need an accessible AD controller with :389 or :636 listening, and an account that can connect to that with permissions.
2) As long as DNS resolves, AD will make its way through the site-to-site VPN. Your routes have to resolve. Make sure you can map a drive, or a similar task like RDP. You can even use AD to handle RADIUS authentication at the remote sites, which is pretty sweet.