The format that you are asking about is incredibly rare and on Mac it's usually associated with an application called Xara Design, which is an image editor that has nothing to do with YAMAHA XGworks, which is a music a tool.
Since I can't find a Mac alternative for your application, I recommend a different solution: use a Windows emulator for Mac such as WineBottler or Boot Camp to install YAMAHA XGworks directly on your Mac, and open your files that way.
