How come I can't shoot

Hint 1 of 4:

Do you see a gun in your inventory (page 38) ?

Hint 2 of 4:

Have you purchased any ammunition for the gun ?

Hint 3 of 4:

When the fighting starts, you must click on the gun icon.

Hint 4 of 4:

The gun icon is above scene, in middle of screen (page 48, top).