You may need to do this to add the unofficial
Make a folder called "Pwnage" on the desktop. Download PwnageTool 4.1.3 into the folder.
You will also need to get the minimal 3GS bundle from here. Double click to extract it to the Pwnage folder.
Double click to mount then drag the PwnageTool icon into the Pwnage folder.
Right click the extracted PwnageTool app and choose Show Package Contents from the menu that appears.
Navigate to Contents/Resources/FirmwareBundles/
Copy and paste the iPhone2,1_4.2.1_8C148a.bundle from the Pwnage folder on your desktop into the FirmwareBundles folder.
You can now launch PwnageTool from the Pwnage folder on your desktop and the application will detect your new firmware bundle.