Ok... our apologies, but it seems that the umods were released with the ".UMOD" extension, rather than the ".UT4MOD" extension.
But... if you have downloaded the umod version of 3.7, and cannot install, please simply rename the file's extension to .UT4MOD and you should be good to go.
Not entirely sure how we missed this... one theory is that if the person(s) testing the umods already had made the "umod to ut2004" association at an earlier time, then it would have fired right up for them.
Anyway.. we apologize for any issues, and please note that the file, and the mod, are just fine if you rename correctly.
--UPDATE!! Version 3.75 umods have been released, and they do include the checkbox option during install ... soo, if you are installing Chaos for the 1st time, this next part is a non-issue. Just install 3.7 1st (of course), and then when you apply the 3.75 umod, you will be given this option (thus no need to do it manually). --
In the past we had a checkbox option during umod installation for whether or not you wanted our mutator to show up in your available mutators list in regular UT2004, or not. We overlooked that in 3.7 umods, but you can still add the Chaos mut to vanilla UT by doing the following (from zip install section of manual):
Using the Chaos Mutator w/o Activating Chaos Mod
Chaos comes in both "mod" and "mutator" form. To use the ChaosUT2 mutator in normal UT2004, without activating the Chaos mod, simply open your UT2004.ini file, found in your UT2004\System folder, and add these lines in the [Core.System] section:
We apologize for any trouble these issues may have caused, and are still looking into it further, along with whatever actions we may need to take (ie: If the umod extension is already incorrect on various mirrors, renaming it isn't really feasible at this point. Posting the quick-fix may be our best route. Having a full umod version of the upcoming 3.75 patch perhaps is also an option. And so on.).