Flash Screens is new, and still very much in experimental mode for Mach 3. As we are still investigating these screens, we can not offer any direct support, however the current talk about these format of screens is found here http://www.machsupport.com/forum/index.php?board=17.0 . It is not expected that flash screens will be editable by other users, and that the screens will be made available for others to use, but not edit. If a user would like a customized screen they will need to use the conventional screen designer, create their own Flash screen , or commission someone else to do it for them.
Mach 3 now has the ability to open a .swf file as well as the traditional .set files. SWF file are native to the Adobe / Macromedia player. There is many different softwares available that can edit and create these swf files. Two that come to mind are Adobe Flash http://www.adobe.com/products/flash/flashpro/, and Swishmax http://www.swishzone.com/index.php?area=products