There isn't really much more I can tell you than what is in the read me file.. It's been awhile since I made a osx wallpaper and don't really remember if I had to do anything special to make it work. The worst part of it I remember is slicing the wallpaper into 12 equal sections and then making sure to save them as 256color .pcx files.. Other than that all I can think of is maybe your having trouble running ucc from the DOS prompt. When you bring up the DOS prompt you will need to CD to the directory where your unreal tournament system folder is. If you have it installed in the default DIR then you would type in cd C:\UnrealTournament\system and hit enter, at the prompt type in ucc make and it should start building your wallpaper as long as you have edited your unrealtournament.ini in the   [Editor.EditorEngine]  section with the proper lines

    EditPackages=osxBackgroundChanger
    EditPackages=name of your background

good luck!!