The actual admsXml code is maintained by the QUCS project and is available at GitHub.

Information on how to compile and install admsXml for Linux or Cygwin is available on the GitHub page. For MS Windows users admsXml.exe is available for download here. You may copy admsXml.exe to your MSYS2 setup into the folder msys64\mingw64\bin, if 64 bit compilation is intended.

More information, though partially outdated, is obtainable from the ngspice web pages.