Did you remove the SD card? Because is you have an empty SD card, the SD card takes priority.
From: Homebrew Channel - WiiBrew
Notes: At this time, loading applications from USB is considerably slower than loading from an SD or SDHC card. When reading the applications to display from the "apps" folder of a usb device, an SD card icon will appear even though it is loading from USB. If you have an sd card inserted at the same time as a USB storage device, the SD card takes priority and apps from the USB device will not be shown even if there are no visible apps on the SD card.