You enter through the back of the atrium - ie where the players go in - you go straight upstairs. Its a nice little area - no queues for bogs and definately warmer in the concourse than the rest of the ground for winter matches. Food and drink queues are bad there though.
You can of course enter through any turnstile and just walk through the corporate area - there is no restriction on this as long as you dont try and get into the lounges - although I imagine that wouldnt be too difficult either if you had your wits about you.