Hi Paul,
The first thing I would try is downloading a fresh copy of Premiere Pro 5.5 and trying to install from that: Download Adobe Creative Suite 5.5 products.
If that doesn't resolve the issue, this forum thread shows some fixes for a similar error: Illustrator CS5 re-install error: Install.app can't be opened. The correct answer doesn't apply, since you are on 10.8 not 10.7 (though checking Gatekeeper is always a good idea to ensure it's not blocking something). However, some of the other suggested solutions, such as checking the installation path in Terminal, may be applicable.
Let us know if any of that helps.