You seem to have some basic confusion here.
The WindchillDS control panel has nothing to do with running the web (i.e. HTTP) server for Windchill. I don't actually know what WindchillDS might use HTTP for itself, but it is not to server Windchill content via HTTP, i.e. it never serves as "the web server" for Windchill.
WindchillDS is an LDAP server it serves to store/provide user, group, and credential information and some Info*Engine configuration information for Windchill. Customers are free to use additional LDAP directories as well, but WindchillDS is generally still used to contain all Windchill-specific groups, since Windchill usually does not have write access to corporate LDAP directories.
As for starting HTTPServer (Windchill's bundled Apache web server), that's done the same way as for all previous versions of Windchill or for Apache on Windows in general (apart from the Ant wrapper for service installation). In particular, I'd verify that the server will start by running HTTPServer/bin/httpd.exe (either directly or via the shortcut in the Start Menu Programs folder created during Windchill installation) -- without the service running. If that doesn't work, the service won't either. If that does work and you want to run as a Windows service, then fine -- install the service and then start it. Installing the service doesn't immediately start it. The starting and stopping of this service works the same as with any other Windows service.
As Patrick noted, running Ant has certain pre-requisites in terms of environment variables. If you're running Ant on another machine than Windchill (e.g. configuring HTTPServer on a remote machine), you have to deal with these directly. If, however, you're running Ant on the same machine as Windchill (and from an account that has access to the Windchill files), then it's easiest to run Ant from a Windchill shell. The easiest way to get a Windchill shell is from the shortcut provided in the Start Menu Programs folder for Windchill.