As I'm getting more and more into developing Flex 2 apps, I am running into silly little annoying problems and am going to start blogging them so I can find the solution when I forget and it happens again.
Problem: The ColdFusion Wizards in the Flex Builder 2 require you to connect to the CFMX server using RDS in order to get table information. In my case, I was getting to the screen where you choose an RDS server and datasource - but and the datasource box was empty. I knew there was a valid datasource at that location, and Flex Builder wasn't complaining that I'd given it a bad RDS password.
After a bit of troubleshooting, I found the following in CFMX's exception.log file: "File not found: /CFIDE/main/ide.cfm". This essentially means I didn't enable RDS when I installed CF. Whoops.
Solution: Since main/ide.cfm isn't an actual physical file, you have to tell CFMX that you want it to intercept calls for this file and handle them for you (enabling RDS). This is done via an entry in the web.xml file, typically located in [cfmxroot]/wwwroot/WEB-INF/. Just open up the file and do a find for "RDS". Two entries should be in there, both commented out. Simply uncomment them and restart CFMX.
As a side note, if Adobe's listening, it would be nice if Flex Builder were smart enough to tell you when RDS authentication fails or RDS isn't enabled on the target server.