Original here: http://www.insanelymac.com/forum/topic/36848-how-to-convert-mac-demos-to-full-games/
http://www.macgamefiles.com/detail.php?item=18034
Obtain v3369 Patch and install to the demo folder (you will have to manually choose where to install patch to)
ut2004-macpatch3369-2.dmg.bz2
Install UT2k4 on a PC (ANY PC will work, you won't be running the game)
->OVERWRITE EVERYTHING WHEN ASKED EXCEPT DO NOT OVERWRITE ANYTHING IN THE SYSTEM FOLDER<--
Make a plan text file with your cdkey in the UT System folder. To do this, type your cdkey in a blank document in Textedit and select "make plain text". Save as "cdkey" and make sure to deselect "if no extension is provided use .txt". -If Unreal gives you a message that your key is invalid, select the cdkey file and go to "Get Information" and "Name and Extension". Make sure there is no extension provided. -Make sure you rename the application Unreal Tournament 2004.app and rename the executable in /Contents/Mac OS/Unreal Tournament 2004. Failure to remove the "demo" from executables may result in a "This application is damaged" message when trying to launch.
I did this but when I double click Unreal Tournament 2004.app on El Capitan I get a message "PowerPC apps are not supported anymore" but I can just double click the alias in /Contents/Mac OS/ and the game runs.
Any ideas how to fix this?