You enter by clicking the double arrow on the ground (move your mouse around the image if you don't see it). Should look like this: http://i.imgur.com/mrl11c8.jpg
I'm told it doesn't work on the new Maps beta, so if you're on that you'll have to opt out to see.
I can confirm it doesn't work on the new maps beta - that's what had me confused. Incognito mode worked a charm though. Great easter egg and thanks for the help viewing it
I'm told it doesn't work on the new Maps beta, so if you're on that you'll have to opt out to see.