@mstan — sorry that you haven't received a response.
NodeBB doesn't save the Mongo database anywhere specific on the file system, it would be wherever the insulted MongoDB install saves it's data.
Which version should I use?
v0.8.x confirmed working on Node v0.12
v0.9.x works on v4 and probably v5.
I'm install NodeBB v0.8.2.
A recent version of nvm.
For node.js I use node 0.12.x
And so far works well.
For details you can inspect package.json of NodeBB : https://github.com/NodeBB/NodeBB/blob/master/package.json#L88