Currently Being ModeratedDec 3, 2012 1:29 AM (in response to Trip)
There is a link to the Trial version in this support article:
You can download the Trial Installer from there; make a backup copy and keep it safe, once you have it! This article also describes, how to reinstall. Download the Aperture 3.1 Trial.
You may have an invalid ProApp System ID file, see my answer here, on how to fix that: