Giulia, coud you please post a crash log of the installer with the exact error message? The first 50 lines only please - just paste the text into the answer, no screenshot.
If the crash report contains a line like this:
PKInstallPackageIdentifier=com.apple.pkg.iLifeFaceRecognition, NSUnderlyingError=0x7fe76d67ec20 "The operation couldn’t be completed. CSSMERR_TP_CERT_EXPIRED"
Then you need to temporarily set the date in your system preferences "Date and time" to a date before March 22, 2012, before you run the installer.
You can set it back later.