To make things clear the U in 4.3U refers to the region of the console where it was made to function in in this case:
U- USA
J- Japan
E- Europe
K- Korea
The problem is your wii is an australian version and letterbomb only supports 4.3U/J/K/E.
Maybe you can use the DNS Trick to get the homebrew channel on the Wii but I am not really sure if will work but you try it at your own risk.
Here is a video how to this trick.