As far as I know, you have to use an older version of Blender to get the .mesh script (Ogre format?) to work. Not sure which version to use, maybe v2.49. But I have successfully exported .mesh.ascii from XNALara and imported into the newest version of Blender with no problem. Some of the models come in a .mesh.ascii format already, so usually XNA isn't even needed.. I'm using a PC so I have no clue how all this would work on a Mac.
Try this link for the Ogre Tools: http://www.ogre3d.org/tikiwiki/BlenderImport
Edit: Found more info here: http://www.ogre3d.org/forums/viewtopic.php?f=1&t=66545