Thanks, but I've done that. (please let me know if I should be starting my own thread; I don't know what the etiquette is; I'm not meaning to hijack it)
The problem is my old desktop just died - it looks like the hard drive is fried. I have time machine backups on an external drive, which I have accessed from my Macbook Pro which is running Yosemite (and iphoto 9.6; I had already updated my library on the hard drive of my laptop). The desktop was using an old version of iPhoto, which is why I needed the upgrader. I downloaded the iphoto library ugrader as instructed and ran it on the library in the most recent backup folder on the hard drive, after which it gave me a popup that said "Your library can now be upgraded by the current version of iPhoto. Do you want to launch iPhoto? However, when I try to launch iPhoto either from the popup or from the application folder (to a make sure it is 9.6) it brings up the same error pop up that says I need to prepare it, with the same link to the upgrader download which I have already installed and run.
Again, apologies if I'm hijacking the thread; let me know and I'll start another one. It appeared to me that it was the same problem.