In the process of moving photos that were originally on an older Mac onto my current Mac with Mavericks and iPhoto 9.5.1. When I first opened the library there weren’t any images showing. To help in this situations Apple has a Library Upgrader available. The upgrader is for iPhoto libraries that you want to use from iPhoto ’08 (version 7) or earlier. iPhoto will still go through the upgrading process when you open the library after that. This worked for a couple different libraries when I used the upgrader.
To open a library other than the default one, use the option key when starting iPhoto. There is also a shareware program called iPhoto Library Manager that is useful when you are working with multiple libraries.
Update 17/04/2014: Unfortunately I found the upgrader doesn’t work in all cases and the pictures still don’t appear in iPhoto. iPhoto Library Manager can help in these cases by using the “Extract Photos” menu item. This will search for the pictures in the library and put them in a folder on your hard drive.