I have the same issue with the debugger,
realized that i kept downloading the same player....
gotta download that one....
hope that's what you need,
you will need to insstall flash player 10 debug version.You are facing
this problem because current version of flash player installed on your
system is not The Debug version . If you have the debug version then you
browser cant find it .so solution will be to install flash player for both
IE and firefox and check which one of them has debug version working.
download the debug version according to your requirement . this page
contains both debug versions for netscape and IE so should install both of
them if in case you need to switch for testing to different browser.Altough
your problem can also be solved by selecting different default browser from
flex builder in which debug version is installed and working.