Instructions
Left-click the "Eclipse" file. It downloads as a ZIP file so extract the files using the Unzip command.
Click "Start," then type "C:\Program Files (x86)\Steam\steamapps" in the box at the bottom of the menu, and press Enter. This opens a window for Steamapps.
Click and drag all the files you extracted from the "Eclipse" folder into the "Sourcemods" folder inside the Steamapps window. Close all folders and windows.
Restart Steam. "Eclipse" will launch.