Unfortunately, it doesn't work on Windows today because our Mercurial support depends on Watchman, but we have made the Hg package for Atom that we use and develop at Facebook publicly available as part of Nuclide: https://atom.io/packages/nuclide.
Previously, we offered individual features of Nuclide as their own package, such as
nuclide-hg-repository. However, due to performance issues with installing a collection of Atom packages, we decided to release Nuclide only as a single, monolithic package. This has solved a number of reported installation issues.