There are two Windows downloads, the straight application and the application bundled with Mono so you don't have to install .Net 3.5. I recommend the download that needs .Net as it looks better, has slightly better performance and is much smaller.
To run JuggleSim simply download then unzip file. Double-click 'JuggleSim.exe' to run program.
JuggleSim (requires .Net 3.5 or later or Mono 2.8 or later) [96KB]
JuggleSim (bundled with Mono Runtime) [5.33MB]