In Flash Professional, the AS Linkage should match the class name. You can right-click from the library and "Edit Class", then select Flash Builder to edit the class.
Test the AS Linkage to assure your class package is correctly referenced from Flash Professional to your code.
Views constructed in Flash Professional will instantiate and add children. If you drag your "Ball" movie clip from the library to the artboard, at runtime the "Ball" class will be instantiated and added to stage.
If the symbol requires no programmatic implementation, then there's no need to create the class.
As per size, there's no magic to the symbol's class definition - Flash Pro automatically creates the class for you. You won't know it's name or how it's been defined when the compiler references the class.
This question could be refined to better understand your objective, or specific issue you are experiencing.
If your desire is to code in Flash Builder, than use Flash Pro to design and layout your views, and export SWC from Flash Pro. In Flash Builder, use the [Embed] meta tag to reference the symbols.
If your desire is to code in Flash Pro, you can still use Flash Builder when editing code.
The "?" does not mean there is an error. It's a warning. You might see it all over the place from the default package - things like int, string, Number, MovieClip may show a "?" on the left by the line numbers because the playerglobal.swc from Flash Pro needs corrected in your Flash Builder's .actionScriptProperties.
Flash Pro's compiler should resolve those references and build your SWF; however, editing in Flash Builder may be tedious without code completion, or simply seeing so many "?" by line numbers.
I'd suggest:
- Create your FLA in Flash Pro.
- Establish a base class, or give a AS Linkage class name to a symbol.
- From the library, "Edit Class" in Flash Builder, and let Flash Builder stub the project in your workspace referencing Flash Pro's src.
If you see "?" from default package classes, update your .actionScriptProperties by replacing the playerglobal.swc:
<libraryPathEntry kind="3" linkType="1" path="${FLASHPRO_APPCONFIG}/ActionScript 3.0/FP10.2/playerglobal.swc" useDefaultLinkType="false"/>
If your symbol has no AS Linkage, you cannot reference it using ActionScript, unless you can assure access via a base class such as calling objects as MovieClip. This would not be recommended.