Here is an old repost of an answer given for fixing the umod file association problem. Also note that "old" umods may not be installable in the latest version of UT.
Change the file association manually using Windows Explorer. Click on View | Folder Options then select File Types tab. Scroll down the list until you find the Unreal Module file type, then click the Edit button. Select the "Install this Unreal module" action and if this action doesn't show in the list, then add it. Select this action and click on Edit, now change the path so that it points to the setup program in your Unreal tournament folder. The command line must look like this:
C:\unreal tournament\system\setup.exe install "%1"
Note that the
install "%1"
you must type in, the first part you find by browsing your drives.
I found this out by checking at this site:
http://www.planetunreal.com/umodwizard/
It refers to Unreal there,but I think you could insert the neccessary changes for UT.