The document is referring to the solution statement that ships with the Interactive Statement Solution Accelerator.
However, if you have created your own Interactive Statement project, you should still see a DocumentMain.mxml in the src\(default package). That is, if you created an Interactive Document Project.
First you need the Flash Builder plugin for IS - installation instructions are on page 22 of the Install Guide
Second create the project as a IS project:
- In Flash Builder choose File -> New -> other
- In the dialog, choose Interactive Documents, Interactive Document Project
- Click Next and give it a name
- Click Finish
You should now have a project with the DocumentMain.mxml created for you:
You may also want to look at the documentation for instructions on how to build and test an interactive statement template using the Asset Placement plug-in for Flash Builder.