Update/Map Pack on Steam?

[MyLeftFootIsGone]

Heya Folks.
I am sorry if this has been posted before, Im quite new here so I'm a noob to using the search botton, made no sense to me

Anyway, I was just wondering whether the Steam Version of Mirror's Edge automatically update? Because I am unable to update my version of it, it says that I have to reinstall it.

Also, if I buy the Map Pack, will it work on the Steam Version? Or will I get the same error?