Way back in The Relationship between RAD 6 and WAS 6, I talked about test environment servers in RAD 6, a feature from WSAD 5.x. According to comments on that posting, there seems to be some confusion about how to create a test environment server for WAS 6.
I guess this is not obvious (enough), but simply put, WebSphere 6.0 Server is the test server; see Creating a WebSpherev6.0 server. RAD distinguishes between a WAS 5.x Test Environment server, used for testing in RAD, and a WAS 5.x server for publishing, used for deploying to a WAS install outside of RAD. The WAS 6.0 server option can be used for both (which I don't think is actually explained anywhere).
Also, be sure to create a separate profile for each of your test servers, so that you can admin them independently; see RAD 6: How to Create a Server. Those instructions are a little out of date now; I think the latest versions of RAD now have a menu choice for running the profile wizard. Anyone want to add a comment on what that menu choice is?