Do you have "Stuffit Expander"? The "sitx" file is a stuffit file that for some reason is not opening properly (if it's giving you random text). If you have stuffit expander you can drag it to the application and force it to open the file and expand it. Most likely the operating system doent know what application to open the file with, so it tries in some text editor and you get the garbled text.
FYI Microsoft discontinued windows media player for the Mac, and instead has been distributing the Flip4Mac quicktime plugin software to enable WMP playability on the mac.
Alternatively, you can use some good freeware media players out there such as "VLC":
http://www.videolan.org/vlc/
I use VLC a lot, because it will play just about anything, regardless of whether or not you have only part of a file, or a corrupted file, it plays what it can whereas other players will shut down on you. Another good one is "MPlayer" (google it).