Originally with my stock HBK2 I had exactly the same problem. No way to get it balanced with the stock 1000mah battery... I assumed that they expected that you'd have a bigger motor and battery... So I added two big washers (about and inch and a half diameter) to the little platform in front of the motor, which did balance it much better...

I like Dawg's idea of extending the battery tray - maybe with a popsicle stick or something.

Now that I've upgraded just about all of the components and have a 1800mah battery - it balances very well without any additional weights!

Eventually (after I learn to fly!) I'll add my small wireless camera (and 9V battery) too - all of which will have to be centered...

Sort of a pain that they don't make it balance correctly right out of the box!