The simple way to get good fill flash in most situations is as follows.
Put the camera in M mode.
Manually set the shutter, aperture, ISO for the correct exposure.
Set the Flash Exposure Compensation to around -1
Turn on the flash
Shoot
Fill in flash works by adding a little bit of light to the shadow areas of the available light in the scene which is why you need to set up the exposure in manual mode. In Av or Tv the camera might decide to use the flash to light the scene instead.
The * button is for exposure lock, if the flash is switched on it will fire the flash to work out the exposure and then lock it so the same power is used when you take a shot. This is useful for when you want the flash exposure to not come from the scene you compose in the viewfinder because it would give a false reading. It's not something I've ever need to use as Flash Exposure Compensation will basically do the same thing.