Dark Interval: Part I contains 4 new chapters - Prologue, Welcome to City 17, Manhack Arcade and Kleiner's Lab.
How to install:
Once downloaded, use 7zip to unpack the archive.
Place the 'darkinterval-release' folder in your \sourcemods\ folder.
Enable '-upcoming' beta option for Source SDK Base 2013 Singleplayer. This can be done via Tools->Source SDK Base 2013 Singleplayer->Settings.
Everything should be ready - now launch the mod. If you see the green menu with the Citadel, you're OK to go.