I don't think the program supports the import of Flash files just the export (in version 10 at least) if you have the MPG4 codec.
If you do a web search you'll see there are a number of free video converters. You may want to try one of those to convert the flash a format the program will accept.
I use Super C, but I don't deal with Flash files so not sure if Super C handles them and I'm not at my home computer to open it up and see.