Instructions
Press the left joystick up or down on your Xbox 360 controller to navigate to the "Games Marketplace" section of the main dashboard, then select "Explore Game Content" within.
Scroll up or down to the "Add-ons or Extras" section and then press "A" on your game controller with "Add-ons" selected.
Scroll to the "Titles A-Z" section of the "Add-ons" menu, then press "A" on your controller.
Scroll over and select "F" from the alphabet listing. This will cause only games that begin with "F" to be displayed.
Scroll down to "Fallout 3" and press "A" on your game controller to select it.
Highlight the DLC add-on you would like to download and press "A" on your controller.
Highlight and select "Confirm Download" on the download prompt that appears. If you do not currently have enough Microsoft Points available for purchase on your account, you will be prompted to purchase more.
Highlight and select the hard drive device you would like to download the DLC mod to. This will cause your Xbox 360 to automatically download the selected content.