DXSimplifySurface is available as an "outboard" module, which can be used with DX version 2.1.5 and later versions, as well as a "runtime loadable" module, which can be used in DX version 3.1 and later versions.
Catenate the above files to your "modules.mdf" file and be sure to use the option "-mdf modules.mdf" file on the command line of dx, or set the environment variable DXMDF appropriately.
Use the option "-modules <directory_of_executable_modules>" or the environment variable DXMODULES to indicate the directory where executable files are located.