Jun 11, 2008 - Cannot change Output Folder in Flex Builder
Main application must be in the list of application paths

The problem: you're trying to get a Flex Builder 3 project going, and in trying to change the Output folder you're getting this amorphous error stating:

Main application must be in the list of application paths

What this means is that there are no MXML files tagged as runnable, and you can verify that by going to the project's properties->Flex Applications, and you'll probably see a folder with no files listed (see the image below).

The solution:

1. Pull up the project's properties
2. Click on the Flex Applications settings
3. Click on the Add button
4. Select the main MXML file for you application.
5. HERE's THE TRICK. If you then click on Flex Build Path to change the output folder you'll still get the error, because the change hasn't been published yet. So you need to hit the OK button first.
6. Now pull up the project's properties again, and you should be able to change the output folder under the Flex Build Path section.

Possible Causes:

• After installing Flex 3 and in setting up previous Flex 2 projects you may have moved the main MXML file to a new location (e.g. by default Flex 3 creates a src folder, where as Flex 2 stored files in the root folder).
• You renamed the main MXML file outside of Flex Builder.

There's a JIRA bug that was posted about it on one of the Flex 3 Beta's http://bugs.adobe.com/jira/browse/FB-8220.