So within Mesoamerica I only know of two contexts in which mercury was used. The first is a very recent discovery of mercury found within a tunnel leading to three chambers at the city of Teotihuacan. We don't know the context of where it was found, how much of it was found, where in the tunnel it was located, etc. So I cannot really say much about it.

The other location is within the Maya region. Several burials (Austin 1994) and ritual offerings there had been found with vessels containing liquid mercury. Within Maya culture at the time, and within Mesoamerica, "mirrors" were highly prized. These "mirrors" consisted of highly polished stones and were coveted for their reflective capability. They were most often used as decoration for costumes. Now, since so little mercury has been found and it is highly reflective, that is where the interpretation that it may have been a mirror comes from.

It could be simply that it was a neat substance and the people who buried these individuals decided to use it as a grave offering.

Austin, Patricia A.

1994 Mercury and the Ancient Maya. Unpublished Master’s thesis, Department of Anthropology, Trent University, Peterborough, ON.

The Roman Pliny discusses mercury in his Natural History circa 78 AD. Unusually, he has no story about how it was discovered. Normally these discovery tales he writes are probably fables, but he does seem fond of them. It's a shame he doesn't have one for the discovery of mercury.

He claims mercury is found in veins of real silver, knows it is poisonous, and says it was used for both gilding and separating out precious metals. Pliny also seems to have some idea of its density.

Source: Pliny, Natural History

In the Friday free-for-all thread, /u/Mictlantecuhtli brings up liquid mercury being found in Mesoamerica: http://www.reddit.com/r/AskHistorians/comments/33pnk2/friday_freeforall_april_24_2015/cqnn6js Might hit them up for more!