You can't. When someone compiles a game, their source code is translated into machine language.
If you want to learn how to program, then looking at another person's source code is probably not the best way to learn. There are guides and demos located at Dream Makers (http://www.byond.com/developer/) that are specifically designed for you to learn from, which are a much better alternative.
Well, in most languages it is. BYOND is converted into bytecode, which is different than machine language in that it has to be interpreted by a virtual machine.
If you want to learn how to program, then looking at another person's source code is probably not the best way to learn. There are guides and demos located at Dream Makers (http://www.byond.com/developer/) that are specifically designed for you to learn from, which are a much better alternative.