Instructions
Download the mods you want to add to Washington D.C. Modifications can be downloaded from websites such as ModTheSims.info, SnootySims.com or TheSimsResource.com. Once downloaded, the zipped mod file will appear on your desktop.
Extract the mod file by right clicking the mod icon of your choice and selecting "Extract to Desktop."
Open "My Computer" then "Program Files." Navigate to "Electronic Arts" then "The Sims 3."
Open "Downloads" if you have "Sims 3: Ambition" installed, or "Mods" then "Packages" if you don't.
Drag the mod file from your desktop into the correct folder.
Open "The Sims 3" and select "Washington D.C." when prompted. Once the city loads, the modifications will work.