Does Steam version of RAGE contain Sewer Missions?

You have to find the sewers first. The job board postings happen after you've gone through a sewer once, and the posts are only for sewers you've cleared. There are quite a few sewers scattered around. Look for manholes.

Source: I played the Steam version.

Small spoiler! The first sewer:

is near the Ark you come out of, under a bridge, and has a guy standing next to it.