tried to test xbuf loader with this simple class:
import com.jme3.app.SimpleApplication;
import com.jme3.light.DirectionalLight;
import com.jme3.math.ColorRGBA;
import com.jme3.math.Vector3f;
import com.jme3.scene.Spatial;
import jme3_ext_xbuf.XbufLoader;
public class testXbuf extends SimpleApplication{
@Override
public void simpleInitApp() {
flyCam.setEnabled(true);
assetManager.registerLoader(XbufLoader.class,"xbuf");
Spatial xbuf_scene=assetManager.loadModel("Models/Models/Players/Doha/Doha.xbuf");
rootNode.attachChild(xbuf_scene);
/** A white, directional light source */
DirectionalLight sun = new DirectionalLight();
sun.setDirection((new Vector3f(-0.5f, -0.5f, -0.5f)).normalizeLocal());
sun.setColor(ColorRGBA.White);
rootNode.addLight(sun);
}
public static void main(String[] args) {
new testXbuf().start();
}
}
but got some errors 
SEVERE: Uncaught exception thrown in Thread[jME3 Main,5,main]
java.lang.NoClassDefFoundError: com/google/protobuf/ExtensionRegistryLite
at java.lang.Class.getDeclaredConstructors0(Native Method)
at java.lang.Class.privateGetDeclaredConstructors(Class.java:2671)
at java.lang.Class.getConstructor0(Class.java:3075)
at java.lang.Class.newInstance(Class.java:412)
at com.jme3.asset.ImplHandler$ImplThreadLocal.initialValue(ImplHandler.java:117)
at java.lang.ThreadLocal.setInitialValue(ThreadLocal.java:180)
at java.lang.ThreadLocal.get(ThreadLocal.java:170)
at com.jme3.asset.ImplHandler.aquireLoader(ImplHandler.java:203)
at com.jme3.asset.DesktopAssetManager.loadLocatedAsset(DesktopAssetManager.java:255)
at com.jme3.asset.DesktopAssetManager.loadAsset(DesktopAssetManager.java:373)
at com.jme3.asset.DesktopAssetManager.loadModel(DesktopAssetManager.java:416)
at com.jme3.asset.DesktopAssetManager.loadModel(DesktopAssetManager.java:420)
at mygame.testXbuf.simpleInitApp(testXbuf.java:22)
at com.jme3.app.SimpleApplication.initialize(SimpleApplication.java:220)
at com.jme3.system.lwjgl.LwjglAbstractDisplay.initInThread(LwjglAbstractDisplay.java:130)
at com.jme3.system.lwjgl.LwjglAbstractDisplay.run(LwjglAbstractDisplay.java:211)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.ClassNotFoundException: com.google.protobuf.ExtensionRegistryLite
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 17 more
there are some differences with your PhysicsLoaderTest.java
Spatial xbuf_scene=assetManager.loadModel(
new XbufKey("models/physicsScene2/physicsScene2.xbuf"));
in my case assetManager.loadModel does not accept XbufKey …
I am using jme3_xbuf_loader2.jar.
Should I build my jmonkeyengine from your github fork?