Make sure you've got the latest DirectX update. As of writing, that can be found here. This is critical, just having DirectX 10 or 11 isn't enough; they get updated every now and then. Without this, the games will complain about d3dx10 files missing or otherwise crash.
Make sure you've got the Visual C++ 2010 runtimes download here. Note that this 32 bit version is also right for 64 bit operating systems, as the games are 32 bit executables.
Make sure your game is patched up to one of the supported versions, it will crash otherwise.
Download the renderer from the link at the top of the page.
Unzip the files for the game you're playing to that game's 'system' directory (e.g. "c:deusexsystem"). Note that the files (d3d10drv.dll, etc) and the 'd3d10drv' subdirectory must be directly in the system directory; make sure not to accidentally keep them in the folder they came in in the zip (e.g. 'deusex' or 'unreal_226_gold').
Start the game, go into its video options, and click the the renderer selection button. The game should restart and allow the Direct3D 10 renderer to be picked (make sure to select "Show all devices").