ProM 6 can be downloaded through the ProM 6 download page, which also includes a Windows installer. The latest Nightly Build is available through the ProM 6 Nightly Build page.
ProM 5 can be downloaded either through the ProM 5 download page, or through the ProM page on SourceForge. The latest Nightly Build is available through the ProM 5 Nightly Build page.