Right now it should work in the newest builds of Mesen, which can be found here: https://github.com/SourMesen/Mesen2
Rainbow is still fairly new, so there may be some bugs we haven't squashed just yet, but it should at least boot and run. I use Mesen2 for development, as it has an excellent debugger. The older Mesen project on github is no longer being maintained, as far as I'm aware.