I installed recently the 6.6.2-rc zeos version on Delphi 5 Pro and had the problem below :
On runtime all is ok. Fine. But at Design time when I try to connect my Zconnection (Set connect to True), I have an Access violotion in VCL50... message !
The dll needed for database connectivity. For Mysql that's libmysql.dll.
I suppose it's working at runtime because the right one is in your system path, but a different one is in the path Delphi uses during design time.