It doesn't exist. We already have the technology to check every place that anyone in this thread has mentioned and in most cases already have.
We can see, from space, the location of every ancient ruin through the canopy, so we know the location of every native american ruin in the jungle and therefore it can't be there.
There's no reason to build a hidden city underground, and even if they had done that we have technology to check for mineral deposits that would pick up on the big hole in the ground with a ton of gold in it.
And whoever said it was the ancient Aztec capitol of Tenochtitlan (which was the name if you were unaware). Well...I'm sure the 8.2 million people currently living at that site would be able to tell you that no, there isn't a city of gold all around them, just quite a few slums.
I highly doubt that there is, or ever was, an El Dorado, it's just one of the many tales that were spawned among the discovery and excitement of finding 'The New World.'