Sorry for the necrothreading. I googled for UT2K4 and widescreen, and this is evidently the first hit on the 1st page.
Answer to the original question: your .INI file is buried within your hidden user folder. You don't have to unhide the folder with Terminal or TinkerTool to get there. Simply go to the menu bar, select
Go > Go to Folder... >
then in the box type:
/Users/[your name]/Library/Application Support/Unreal Tournament 2004/System
UT2004.ini should be right inside.
And @saturnotaku. As an otaku, you =