If you uncheck “Generate HTML wrapper file” in your Flex Builder Project ActionScript Compiler options (which is good idea if you want to use SWFObject to wrap your SWF rather than the crud that Flex Builder generates), Flex Builder uses the system default application for opening SWFs when you run/debug your project.

If you get the “Flex Builder requires a more recent version of the Flash Player. You might need to install the Flash Player or reinstall Flex Builder.” error, the solution is to set the system default application for opening SWFs to point to the latest debug player. To do this on Windows right click a SWF in explorer. The latest debug player may be in the following location: C:\Program Files\Adobe\Adobe Flash CS3\Players\sa_flashplayer_9_debug.exe, otherwise download the latest Projector content debugger and point to it.

Here is some more Mac specific help