Filename.current_dir_name is likely to be just ".", it indicates the (location-agnosti) way to refer to the current directory on your system, to make it easier to write code that is portable on other operating systems with different path conventions.
You are probably looking for
Sys.getcwd : unit -> string
which when called returns the current working directory (as the `pwd` command would do). It may of course change over the time of your program execution, for example if you called Sys.chdir. There is also
Sys.executable_name : string
(or Sys.argv.(0)) which contains the path of the executable, but it may be a relative path -- it depends on how the program was invoked.