ragesteam
I have not encountered any Sewer Missions on job boards while playing through RAGE. The Wiki only describes the Wasteland Sewer Missions as X360 DLC. However in game properties in Steam I can see "Rage Sewers DLC". Is it something different than the X360 DLC? Or maybe I need to do something to activate this DLC?
You can tell by the SteamPlay logo. The SteamPlay logo means that a game is playable on both Mac and PC, and just a Windows logo means it's only playable on Windows.
This is the SteamPlay logo: 
This is the Windows only logo: 
In other words, yes.
The problem of region restriction and low violence content has already been discussed here. Basically those restrictions are applied based on who purchases the game, not who plays it. You need somebody in a non restricted area to gift that game to you. Alternatively, you could spend your holidays in one such country and purchase the game from there with a local payment method (apparently you can buy prepaid cards from gas stations, or at least that's what Redditors say to people who have no credit card; YMMV).
Best Answer
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: