Instructions
Go to Anticyclone's website, at anticyclone.be and download Active Camera 2004. Extract the downloaded file with Win Zip or Win RAR.
Double-click the ".exe" file within the extracted folder to install the modification.
Start up "Flight Simulator 2004." Once in game, you can hold your middle mouse button and move the mouse to look around your cockpit. Pressing key on your number pad will allow you to move around the plane. 8 moves you forward, 2 moves you back, 4 moves you left, and 6 moves you right.