The .mo files, the binaries, are the result of compiling the .po files. So, you should better work on the original .po files  The reason behind the .mo files is speed: binaries are much faster to parse when running the application.
Translation, as well as other parts of the development, is much easier on a Linux desktop. There, you could use 'kbabel' or sth similar that does all the work for you.
You could also take a look at the info page of 'gettext', in the GNU documentation. It is rather lengthy (GNU's have that), but -trust me- there is good reason behind all this complexity.
 You could reverse-compile the mo ones to get the po's. But that is not recommended. Added after 5 minutes:
The KDE project also has a very good howto on translation:
.. since their technologies are similar to ours, most of that Howto applies to a2billing as well.