Tailwinds, many thanks for confirming that the program works with .Net Framework 2.0 and for your kind comments! Appreciated!
viking9, the decoder file is squawk_codes.txt, a simple text file that lists the descriptions of the various squawk codes. You can edit this file yourself as long as you maintain the format described in the comments at the top of the file (a comment begins with ';' or anything non-numeric for that matter). The program, SquawkBox.exe, expects to find squawk_codes.txt on the same directory as it was started from. The idea was that users in countries other than the UK would customise (or is that customize) the file squawk_codes.txt for their particular country because the squawk codes are country dependent.
radarphil, don't worry about port 30003. That's just internal jargon used on the forum for the 'location' at which RadarBox outputs its flights to which can be picked up by add-on programs. SquawkBox handles the reading of this data by itself and you don't need to do anything more. I'd referred to port 30003 to distinguish it from the flight data output to recorded flight files. These are created by selecting "Start Recording..." from the menu. SquawkBox can also read these files while they are being created.