How to run the 3.1 master?

There is none, you have to open the SDK project in the SDK or NetBeans and run it or run the build-zip ant task in the SDK using ant.

If anyone is wondering, what to do after this -

You can find the jMonkeyPlatform.zip under sdk/dist/

Unzip it, and you’ll fine the jMP lancher under jMonkeyPlatform/bin/

If you are on linux distribution and your jMP update isn’t taking place, read this thread.