The D40 works as SLRs have worked for half a century, showing the view only through the viewfinder. The LCD is for menus, setup, and reviewing pictures you've already taken. The LiveView capability that allows you to use the LCD for composing your pictures is a relatively new feature. Nikon introduced it into their cameras with the D3 and D300, released well after the D40.
Sorry if that wasn't the answer you wanted to see, but that's the way this camera works.
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