First ensure your usb is plugged into the correct port, its the one closest to the edge of the wii as you look at the system from the front.
I also suggest remodding your wii by following the soft mod any wii guide on this site to get your system bang up to date. You can also get the latest versions of the USB loader of your choice from the same guide.
You can find the guide in a moderators signature or use the delightful search feature on this site.
If that doesn't fix your problem report back.