Your own defined presets are stored in your hidden User Library
~/Library/Application Support/Aperture/Export Presets.plist,
as Frank Caggiano said, not in the System library
Macintosh HD/Library/Application Support .
Your User Library is hidden by default in 10.7.x or later - to open it in a Finder window use the "Go" menu from the Finder's main menu bar.
- Quit Aperture, if it is running. Log off and on again.
- Open the user library by using the Finder's "Go" menu and hold down the options-key, until "Library" appears in the drop down menu. Select it.
- In the widow that will open, scroll down to "Application Support" and open the "Aperture" folder inside.
Regards
Léonie