Simple, unzip the file (it's called "AS_F16-FIGHTING-FALCON_FSX.zip") to a place that's easy to remember (e.g. your desktop), then double-click the installer (AS_F16-FIGHTING-FALCON_FSX.exe). It will ask for the email address you used during purchase and the keycode you received after downloading. After that everything will go automatically - it will find your FSX folder by itself and install all files there.
If you don't have the key anymore you can find it by
logging on to your Aerosoft account.