Mac
Directions for using the MAC OSX Folder Icons:
Select the icon file you want to use. Press Apple-I to bring up the Inspector panel. In the upper right of the Information Panel click on the icon once. You should see a gray border around the icon. Press Apple-C to copy the icon. Do not close the inspector panel. Select the file or folder that you want to customize. Press Apple-I to bring up the Inspector panel again. In the upper right of the Information Panel click on the icon once. Press Apple-V to paste the new icon.

Windows
Directions for using Windows Folder Icons:
Right click the Folder you would like to change the icon. Choose Properties from the menu. In the properties dialog, press the "Change Icon" button to open the icon selection window. In the icon selection window, press the "Browse" button. Use the file selection dialog to locate to the folder (Win_HappyHalloween) that contains the icons you downloaded. Choose the ICO file you want to use as a replacement and press "Open". Press "OK" in the icon selection window and on the properties dialog.