If you followed the documentation on how to enable RDS in the web.xml, FlashBuilder should give you a reasonable error. Ensure that you have configured RDS in FB with the correct application root (it defaults to /lcds) where you have data services deployed on yout application server.
Here is the topic for configuring RDS on the server:
Here is the topic for configuring the connection to the RDS server in Flash Builder:
hope that helps