1. Save the downloaded file on your pc, where you can find it later.
2. Open the blender file in which you would like to import the apartment system.
3. File --> Append --> ACS_Apartment Generator.blend --> NodeTree --> ACS - Apartment Generator
4. Play with the input attributes in the modifier stack until you like how it looks.

The system is designed to work with auto smooth. So just right click the model and choose "Auto Smooth". It should look better now!