According to this article "Mars atmosphere is supersaturated with water" & according to answers to this physics stack exchange question conditions do exist on the surface of Mars where fluid water can exist at certain times of the year.
Comments in this question also suggest there's more than enough carbon dioxide in the Martian atmosphere for these organisms, and unlike most plants they can supply their own oxygen needs.
While this article suggests some lichens and cyanobacteria (the organism previosly known as blue green algae) are tough enough to handle Mars.
The Hellas Planitia impact crater & Valles Marineris trench seem the obvious choices of location to try & seed these organisms (if we were going to) for their higher atmospheric pressure (both being around 7 km deep).
If I'm putting everything together properly then everything they need to survive (& even flourish, albeit in a sporadic seasonal fashion) seems to be there & given the nature of these organisms the higher radiation (on Mars compared to Earth) shouldn't be an issue, so what have I missed?