These are the files you need:

http://download.dfbgaming.com:9080/UTDC_2.1.zip

Open up the zip and there should be three files there- IUTDCv12.u, UTDCv21.u and UTDCv21.dll.  These three files should go into your UnrealTournament\System directory.  Once you extracted those files, open up UT99 (or close and restart UT99 if you had it open while you put the files in there) and you should be good to go.

If that doesn't work then they may be using a different version of UTDC.  What's the server IP and I can check what version the server uses.