Check out these two jews harp links:

Jews Harp Guild

Mouth Music

You'll see info on how to play them as well as info on why they're commonly called jews harps in North America and why many regular players still call them that.

And keep trying. Set it in front of your teeth with the diagonal of each bar resting against the edge with enough room for the "twangy bit" to go back and forth, and put your lips around the rest to seal it so the sound only will come through the center. Breathing in and out while plucking will increase the volume quite a lot on good jews harps too.

The biggest problem is that the ones you find in most music shops are cheap and aren't nearly as good as "professional" models. Most of my collection go for around $20 to $40 dollars and each us tuned to a certain key.