the menu on the left side of the screen should be self explanitory. To bomb, you use the symbol with the red arc. It looks like the ranged units attack button. When you press this button with bombers, you should get a red line on the map showing the bomber's range. Note that you can't bomb something unless you have line of sight. This sometimes means you need ground troops close to the target.
If you don't have this line of sight, or if the target is outside your range, then nothing will happen when you click on the target. This might be your problem.