After riding either and getting off again, the mob can't move anymore unless pushed by a player or other mob. And by that, I mean you move them manually—they'll never walk on their own again (until a restart).

14:59:55 [SEVERE] Could not pass event PlayerInteractEvent to MobRider org.bukkit.event.EventException at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:303) at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:459) at org.bukkit.craftbukkit.event.CraftEventFactory.callPlayerInteractEvent(CraftEventFactory.java:175) at org.bukkit.craftbukkit.event.CraftEventFactory.callPlayerInteractEvent(CraftEventFactory.java:145) at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:605) at net.minecraft.server.Packet15Place.handle(SourceFile:39) at net.minecraft.server.NetworkManager.b(NetworkManager.java:229) at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:113) at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:78) at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:554) at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:452) at net.minecraft.server.ThreadServerApplication.run(SourceFile:490) Caused by: java.lang.NullPointerException at com.edwardhand.mobrider.models.Ride.<init>(Ride.java:45) at com.edwardhand.mobrider.utils.MRHandler.getRide(MRHandler.java:39) at com.edwardhand.mobrider.listeners.RiderPlayerListener.onPlayerInteract(RiderPlayerListener.java:80) at sun.reflect.GeneratedMethodAccessor41.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:616) at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:301)

Try the dev build that has ocelot support:
http://ci.milkbowl.net/view/mung3r/job/MobRider/