Not all DVD media are Wii-Compatible maybe you changed the brand, also check + and - DVDs (for instance the new DVD-R 16x silver princo doesn't work while the old up to 16x DVD-R white princo work fine).
Remember also to burn at 4x, I use ImageBurn for this and it always does the job well.
The OP isnt dumb hes been doing it for some time. Also He has a mac , maybe you can post a link for imgburn that works on a mac ????????
Personally would suggest using USB loaders and to invest in a SA USB Drive , i dont know the reason you have the error sorry .