There are a few extra steps to get the current version of BPM to work on Windows. I'll work in the future to correct this issue on this branch. But for now you must do the following:

1. I recommend that you Install Ruby 1.9.2p290 from here: http://rubyinstaller.org/downloads/ as this is the version this article was tested on.
2. Download and install Dev Kit from link above and follow instructions here to complete its installation https://github.com/oneclick/rubyinstaller/wiki/Development-Kit
3. Then run gem update --system
4. Next, install execjs version 1.2.4 with command gem install execjs --version 1.2.4
5. Finally install BPM: gem install bpm

If you have already installed BPM and are getting the which.bat error it's because you have the wrong version of execjs installed. All you should need to do is run gem install execjs --version 1.2.4