In the "Project" window, expand the "libs" directory, right-click on the .swc file (the Flex API), and click the "Add to Library" menu item.
You will also have to change all the references to the image files as noted in another post.
I'm a couple of steps ahead of you...I can get the project to compile, but when I run it, it's missing an xml file for the Widget Container. If I figure out that error, I'll update this post or start a new thread.
One thing to keep in mind with flashdevelop, and the primary reason I went back to flash builder, is that it does not support flex modules. Each individual widget needs to be a separate project and the output directory needs to be set to equate the output of flash builder (where the api expects to find things).
This is probably why your widgetcontainer is not being found, it's not compiled and in the correct output directory.