Your signature states
Mac Book Pro, Mac OS X (10.6.7)
You need Mac Book Pro, Mac OS X (10.7.4) to install Aperture 3.3, also an Intel-Mac with 64bit architecture.
To be able to upgrade Aperture the application needs to be installed in your Applications folder on your MacintoshHD, no whrere else, and it needs to ne named "Aperture", not "Aperture 3,0" or something else. Have you checked the file name?
Thanks for your help.
I do have 10.7.4 installed, not sure why I selected 10.6.7!
I had Aperture tucked away in a sub-folder. As soon as I moved it back ino the root of Applications the install worked.