Close Steam completely (check your Task Manager to ensure Steam.exe is not running).
Obtain the GreenLuma files from a trusted source (usually community forums like CS.RIN.RU).
Steam will launch. If successful, you should see the "GreenLuma" watermark or notice your DLCs listed as "In Library" on the game page.
Many antivirus programs flag GreenLuma as a "Trojan" because it injects code into Steam. This is a false positive, but you will need to whitelist the folder.