A lot of people are complaining of errors with USB Loader GX.

A common thing I have noticed you generally get a DSI error when the nunchuck is plugged in - for some reason USB Loader GX doesn't like the nunchuck, especially the older versions.

If you still get problems try using a different version of USB loader, some updates have rendered the software useless/buggy for many users so play around with different versions and see if you can find one that works for you.

And finally if all else fails check your IOS/cIOS and other installed software - they may be old or buggy versions installed - have a check round the forum for help with upgrading your Wii.

USB Loader GX: http://usbloadergx.koureio.net/