I know this may sound like common sense. But have you formatted the SD card to fat32?
What system menu are you on and are you using a SD or sdhc card?
When you say the Wii is not reading a SD card, do you mean while in the homebrew channel? Press 1 and make sure to check SD and not USB.
I know this stuff sounds pretty basic, but it will help with troubleshooting.
Also post a system check.