The Flex API does not support editing data on pre-10 servers. This is because these older versions do not support editing through the REST API which is what Flex (and the other ArcGIS Web APIs) use to communicate between the browser application and ArcGIS Server.
The easiest solution would be to upgrade the ArcGIS Server to 10.
If you still want to do editing with Flex and 9.3 ... I think some people implemented their own server side components that Flex could talk to, but this would be a fair amount of work. In addition, even if you created such a component, it's not likely that the Viewer would work with it. You would probably need to create a custom widget to talk to your custom server endpoint. Again, upgrading to "10" would make your life a lot easier 🙂