I was always trying to do this as well. Not sure you still need it. Couple months i got it to work. If you have the unix version you open Terminal and drop that file in there. Best thing to do is to make a folder on you desktop. It doesnt like space in file names therefor its best to do it there.
Then drop you xml model file also on there, now you see 2 file paths next to each other. Then simply hit enter/return, you could get a warning if you dont have proper java lib installed, click more info and download the java for OSX.
Then try again. The OBJ will be put in the folder where you dropped the xml model file from. It doesnt get a MLT file, so you would need to recreate the material though