ZIP archive contaning Dark Interval: Chapter I. Unpack in Steam\Steamapps\Sourcemods, download/install Source SDK Base 2013 Singpleplayer in Steam>My Library>Tools, make sure to enable "-upcoming" beta option for it, then restart Steam.
ZIP archive contaning Dark Interval: Chapter I. Unpack in Steam\Steamapps\Sourcemods, download/install Source SDK Base 2013 Singpleplayer in Steam>My Library>Tools, make sure to enable "-upcoming" beta option for it, then restart Steam.