Whilst the game both requires and runs through Steam, the mod is not available for download via the Steam store due to legal reasons.
Quote: Is there / will there be Linux or Mac support?
Unfortunately Source SDK Base 2007 does not support operating systems other than Windows. We had planned to move to Source SDK Base 2013 which does support Linux and MacOS and have invested considerable time and effort into porting over to it. Sadly at this time we simply do not have the resources to port the mod to Source SDK Base 2013. Simply put the code changes and numerous unfixed bugs in the engine cause more problems than we can deal with at this time. The next release will still be on SDK Base 2007 but we would still like to port the mod to support other platforms in the future.
Quote: Are you working on a singleplayer mode for the mod?
No, we have no plans to make singleplayer as we do not have the resources or sufficient number of developers to create it.