Files for the Wii port are provided in the port/ directory.
All other changes made while creating the Wii build are provided in non_port_changes.diff, which can be applied using GNU patch:
patch -p0 < non_port_changes.diff
Grab the POWDER source (version 117 at the time of typing), copy the Wii port directory over and apply the patch.
On Windows, I use Cygwin (with the make and g++ packages installed) to build the support files - this should be as simple as changing to the port/linux/ directory and invoking make premake.
The Wii build toolchain is devkitpro and devkitPPC.
Ensure you have SDL Wii and its dependencies available in your libogc directory as per its usage instructions.
Change to the port/wii/ directory and invoke make.
To test, start the Homebrew Channel on your Wii and invoke make run.