What version of the extension developer are you using? I believe ThingUtilities is deprecated, but I can't find any documentation on it, so maybe not.
You may want to try using the EntityUtilities class:
DataTableThing myThing = (DataTableThing) EntityUtilities.findEntity("MyThing", ThingworxRelationshipTypes.Thing);
Can you debug and see if your call to find the entity is returning something? What do the errors say?
We are using Thingworx 6.0 just to clarify. I tried your solution and also I tried
DataTableThing db = (DataTableThing) ThingManager.getInstance().getEntity("myDataTableThing");
your solution returns null because it does not get past my error check. My attempts both return something, but still says unable to get entries: null so its a null object I assume.
I just hooked up the debugger and it finds it and I can see the description. But as I step through I get to where the datatable entries are showing in the variables in eclipse. Its strange because I'm seeing the current row is -1 and the size is 0. But if I look at the context it shows one row with all the fields filled as I'd expect.