unzip the .zip-file into your UT 2004 directory (for example C:\UT2004)

alterntively:

Copy "DKopp2Meshes.usx" into you UT2004 "StaticMeshes" folder;
Copy "DKopp2AnubisM.usx" into you UT2004 "StaticMeshes" folder;
Copy "DKopp2Tex.utx" into you UT2004 "Textures" folder;
Copy "DKoppIIanim.ukx" into you UT2004 "Animations" folder;
Copy "DKoppIISounds.uax" into you UT2004 "Sounds" folder;
Copy "DKoppII.u" into you UT2004 "System" folder;
Copy "DKoppII.ucl" into you UT2004 "System" folder;
Copy "DKoppII.int" into you UT2004 "System" folder;
Copy "ONS-FORTS-AirSovereignty.ut2" into your UT2004 "Maps" folder;
Copy "ONS-FORTS-Outland.ut2" into your UT2004 "Maps" folder;

Start UT 2004 choose "Onslaught" and select D3nnisK's Forts in the mutator selection menu.
Have fun!