I have websites built using the flexviewer, but would like to import them into Flashbuilder 4 in order to enhance them. Is there a way to do this by importing the .xml or config.xml from the compiled flexviewer? Or is the workflow to build custom widgets in flashbuilder and deploy them to the flexviewer?
in Flash Builder 4, Go to File Menu -> Import -> Flash Builder project.
Keeping "File" option selected, click "Browse..." button.
Select flexviewer-2.3-src.zip downloaded in step 1, e.g. "C:\Documents and Settings\username\My Documents\flexviewer-2.3-src.zip".
"Extract new project to:" textbox will be automatically set to location where the project source will reside, e.g. "C:\Documents and Settings\username\Adobe Flash Builder 4\FlexViewer. Do not put it onto your web server - you should separate your code location from your output.
Click "Finish" button. Project will be created and displayed in the Package Explorer window of Flash Builder 4, e.g. in this case FlexViewer.
source http://www.arcgis.com/home/item.html?id=3f6a0bfee48949a88df50bf7686ec72a (April 15, 2011) unzip flexviewer-2.3-src.zip (readme.txt)
You will really want to review the ESRI docs @ Resources.ArcGIS.com
There you will find not only the Source code for the Flex-Viewer, but also complete walk-through for building your first Widget, and how to install/configure it.