See this support article on how to reinstall your Aperture version: You'll find a link to the trial there.
If you orginially installed Aperture 3 from a trial version:
- Go to the Applications folder and delete Aperture.
- Download the Aperture 3.1 Trial.
- Open the Aperture Trial disc image.
- Double-click the "Aperture Trial.mpkg" file.
- Follow the onscreen instructions to install Aperture.
- After the installation completes, choose Software Update from the Apple () menu.
- Install the Aperture 3.4 update from the list of available updates.
Note: If you are using OS X Mountain Lion v10.8.2 or later, you may receive an alert stating that Aperture is from an unidentified developer. This can occur when you try to install Aperture from DVD or the trial download. See OS X Mountain Lion: The app is from an unidentified developer for details on how to successfully install Aperture in this situation.