The first information you probably need to solve your issue is that eZ Publish 4.4, mandatory intermediate step when upgrading from 4.3 to 4.5 (as shown in the upgrade doc : http://doc.ez.no/eZ-Publish/Upgrading ), is available here :
Secondly, eZ Publish is available in two flavors :
[li]eZ Publish Community Project, openly available, cutting-edge version, where both the editor and the eZ Publish Community are contributing. The latest version is available here : http://share.ez.no/download-develop/downloads/ez-publish-community-project-4.2011 [/li]
[li]eZ Publish Enterprise Edition, available through a paying subcription. More information here : http://ez.no/eZPublish/eZ-Publish-Enterprise-Subscription [/li]
I encourage you to visit http://share.ez.no, the eZ Publish Community website, where the very active forums will help you get a solution to your problem, pretty quickly.