Aperture 3.4.5 is the last compatible Aperture version for Mac OS X 10.8.5. If you want to upgrade to Aperture 3.5.1, you will need to upgrade your Mac OS X 10.8.5 to Mavericks, Mac OS X 10.9.
The upgrade to Mac OS X 10.9. is free, and after upgrading to 10.9 you will be able to update Aperture 3.4.5 to 3.5.1 for compatibility with IOS 7.
Is there a reason, why you want to stay with Mac OS X 10.8.5?
Thanks. That makes sense of why I could find no software update to download.
"Is there a reason, why you want to stay with Mac OS X 10.8.5?"
There is one particular open source program (Hugin) that is essential to my work-flow and I was waiting to be sure that it was compatible with Mavericks. Others have confirmed that it works fine. So I will move to Mavericks when I have a spare evening.
In general I am cautious by nature ... I usually let the keen brave people install the latest everything. Then when any incompatibilities have been ironed out I update to 10.x.1 knowing that my essential applications will work.