Overwatch

One must first set up the Framework so that the game can work with .package based content. All mods, and some CC depending on how the creator provides them, are .package files. NRaas mods are no different.

The Framework is really just a set of three subfolders nested properly and a required Resource.cfg file put in the correct place that makes it all work. The easiest way is to download the Framework from the link below to your Desktop or some other location, unzip, and place the Mods folder and its contents into your TS3 user game folder in Documents. The downloadable one includes two small sample mods, NoIntroMovie and NoBuildSparkles, so you can test the setup before adding anything more complex. After testing you can keep or delete the sample mods as you wish. The setup when finished should look exactly like the illustrations provided, plus or minus some variation around the edges for different operating systems.
http://modthesims.info/wiki.php?title=Game_Help:Installing_Sims_3_Package_Files/Setup_and_Files

Once you have that set up and tested, you can add mods to it by downloading, unzipping, and placing the package file for the mod (Overwatch for example) into the Mods\Packages subfolder and then clear (delete) the scriptCache file before starting up again.

@igazor Thanks!