The Digital Extremes site has word of problems with the .umod installer with the Unreal Tournament mutators they released (story). Here's the update:

Hmmm... it seems that the .umod installer is not working as expected. I was playing online and the ammo pickups were invisible which means that the dedicated server is missing a line in it's UnrealTournament.ini file in the /UT/System directory. This problem will occur on dedicated servers for the Volatile Ammo and also the Team Beacon mutators.

To fix this, the following entry should go under the [Engine.GameEngine] heading:

ServerPackages=DE

This change should have been made by the .umod file, but instead it creates a new UnrealTournament.ini file in the /UT directory. Weird. Anyway, making this change should fix this problem for dedicated servers, which is the only place it will occur. In the meantime, Steve is working on making a new .umod file (by hand... ugh) that will install correctly. I guess it slipped by us here 'cause we had the line in our .ini already.