Any library from Aperture 3.0.x must be upgraded to Aperture 3.1 before it can be imported or merged into Aperture 3.1. To do this:
- In Aperture, choose File > Switch to Library > Other/New
- In the dialog that appears, select the library you want to upgrade from the the list. If the library you want to upgrade is not listed, click the Other Library button and use the file selector to choose the library.
- Click the Upgrade button in the "Welcome to Aperture 3.1" dialog.
- Switch back to your main Aperture library.
It is now possible to import the upgraded library into your main library.