Is there a problem with the installer media you bought? If you bought Aperture 3 as a boxed retail version, you can download the Trial version again here:
After you install it, run immediately software update from the -menu. You can unlock the Trial version with the serial number that came with your boxed CD version.
The licence that came with your boxed version entitles you to install it on two macs - one Desktop mac and one portable.