I’m pretty sure Lemur doesn’t have one [visual editor], I would have assumed pspeed, the author, would know about it and he would’ve already told about one. For Nifty, there is only the renderer implemented in jME. Nothing else, Nifty is a separate project. So is Lemur and all the others. jME doesn’t really have an internal GUI system.
For troubleshooting the Nifty editor itself.. well. I don’t know when I have time to try. Here is at least a video tutorial for it: https://www.youtube.com/watch?v=O0rdFwRVYbE. It is the same editor whether you just got it from the guy’s repo or manage to run through the SDK.
Someone seemed to have managed to run it through a modern SDK even Fix SDK Nifty Gui Editor. Perhaps the SDK could also force the Nifty 1.4.1 as a dependency, I assume only Ant projects would suffer from this if any…