I don't think you have to download anything. Simply install Unity and Plastic SCM (e.g. Cloud Edition), and then in the Unity Editor Project Settings for the Editor, set version control mode to Plastic SCM. This will enable the feature that draws icons on top of the assets in the Unity project window and lets you checkin file directly in Unity. As far as I know this is already bundled with the Unity installation. There's an Asset Store page titled Plastic SCM that looks like the official plugin, but in fact, the last time I opened it, it was simply an interactive installer for Plastic SCM.