Terry, that's a nice composition!
The primary reason the sharpness is off in this photo is probably 'camera shake'. The EXIF data embedded in the photo shows that the shutter speed you used (no doubt set for you by the camera!) was only 1/5 second - that's generally too slow for a hand-held shot (I'm guessing you didn't use a tripod for this candid gem!). When the shutter is open for that long, any slight movement is exaggerated in the photo.
A second factor - related to the first - is that the aperture was nearly wide open at f/3.3. This reduces the 'depth of field' - the degree to which the photo is in focus from front to back. Probably not as large a factor as the slow shutter speed, but a contributor nonetheless.
Bottom line: if shutter speeds are much below 1/30 second, consider using a flash or stabilize the camera on a tripod.
Back to your original question: how to use Unsharp Mask to improve sharpness. I would make a duplicate of the background layer, then apply the Unsharp Mask to the duplicate layer; experiment with different values of the Amount slider in particular, sliding way to the right while watching the preview, then backing off to a point where it looks realistic. Not sure the other two sliders will have a lot of impact here, but you can experiment with them as well. Once the USM is applied to the duplicate layer, you can switch on and off the visibility of that layer to see the after and before of sharpening, and you can reduce the opacity of the layer if you decide you've oversharpened.
Hope that helps...