Do you have a title on the SD card? I used an alternate method:
Put the SD card into your computer, and go to Private/DS/Title. You should find a .bin file there. Google for DSi SRL Extractor, and download it. Place the .bin in the same folder as the .exe, and then run:
Code:
dsi_srl_extract.exe --basename=[Game Name] [.bin Name].bin
Replacing Game Name with your game's name, and .bin name with.. the name of your .bin.
You should get a .footer file. Open the [Game Name].footer file with your favorite hex editor. In the hex editor, look for "Root-CA00000001-MS00000008" in the ASCII window. You will find "TWxxxxxxxx-yyyyyyyyyyyyyyyy" either directly, or shortly after that, with the "x"s and "y"s showing your unique codes. All we need is the data after the dash, so, the "y"s. That 16 character long string is your ConsoleID.