I need my serial number from the downloaded copy of Aperture. How can I get it?

Downloaded from where? There is no serial number if you downloaded from the App Store.

if you downloaded from the Apple Site and Aperture is installed you can get the serial number from the Preferences pane of Aperture. If it is not installed and you don't have the email sent to you with the serial number you can try Pro Application Replacement Serial Numbers

if you downloaded from the Apple Store or unlocked the Trial Version, and if you have an Installation of aperture on your Mac you can get the serial number from the About Panel of Aperture:

Aperture's main menu bar > Aperture > About Aperture

The panel will show the Serial number.