Matt from Evolve, wrote a nice article (an how to) which will teach you the way to install and setting up Eclipse and FDT3.
For those that don’t yet know, Eclipse is originally a tool used to program Java. It has things called Perspectives, which could probably be called Plugins, that basically lets you use Eclipse to edit code of other languages while getting all the advantages of using a tool like Eclipse. Eclipse runs on the Java framework so it is available on both Mac and PC. FDT3 (Flash Development Tool version 3) is a tool developed by Powerflasher which is basically a perspective for Eclipse to edit ActionScript code. In conjunction with the Flash CS3 IDE it becomes very powerful. One thing to note, however, is that you can only develop strictly object-oriented code in FDT as it does not work with timelines and all that. It is purely for ActionScript development.