If the mod is a Mutator, remember to add the mutator before you play a game.
If you need to edit the UT2004.ini file it is in the '<UT2004>/system' folder. Edit it using Notepad to add the package files to the ServerPackages in the [Engine.GameEngine] section.
Example, if the new packages are called jcbNewSkins.utx, jcbNewSkins.upl and jcbMutator.u edit the UT2004.ini file as follows:
[Engine.GameEngine] CacheSizeMegs=32 UseSound=True ... ServerPackages=UTClassic ServerPackages=BonusPack ServerPackages=SkaarjPack ServerPackages=SkaarjPack_rc ServerPackages=jcbNewSkins ServerPackages=jcbMutatorIt should only be necessary to do this on the machine acting as the server.
Functions that are server side only, will work without any setup on either the client or the server.
If the package is registered, it will automatically be copied, to any client machine, when that client joins the match. This is OK for small mutators but should be avoided for large packages, such as skin packs.