In case someone also runs into this problem, I though I would post my experience in upgrading to open source version 4.5.5 from open source version 4.5.3 on Ubuntu 6.06 LTS. The install package is zcs-4.5.5_GA_838.UBUNTU6.tgz.

On first attempt, the install script gave the following error message
------------------------------------
Checking for prerequisites...
NPTL...FOUND
sudo...FOUND sudo-1.6.8p12-1ubuntu6
libidn11...FOUND libidn11-0.5.18-1
curl...FOUND curl-7.15.1-1ubuntu2
fetchmail...FOUND fetchmail-6.3.2-2ubuntu2.1
libpcre3...FOUND libpcre3-6.4-1.1ubuntu4
libgmp3c2...FOUND libgmp3c2-4.1.4-11ubuntu2
libexpat1...FOUND libexpat1-1.95.8-3
libxml2...FOUND libxml2-2.6.24.dfsg-1ubuntu1
libstdc++6...FOUND libstdc++6-4.0.3-1ubuntu5
libstdc++5...MISSING
openssl...FOUND openssl-0.9.8a-7ubuntu0.3

###ERROR###

One or more prerequisite packages are missing.
Please install them before running this installer.

Installation cancelled.
-----------------------------------

To install the missing package I used
$ sudo apt-get install libstdc++5

The install script then ran to completion flawlessly.