Some progress on the metaframe file opening problems from the desktop in OS X. I no longer get invalid file errors and the correct server app is opening for the metaframe files. Thank you for that.
However, the following problem still occurs: if you open a metaframe file of one type (e.g., 32-bit) and then try to open one of a different type (64-bit), by double clicking the metaframe file, then it won't open. Also fails to open if you try to open a different metaframe file of the same type.
Note: You can launch the app and open it from the file menu. Or you can select both files and double click one of them and they will both open. So it's not consistent with how it should work yet, but better than before. Open from the file menu always seems to work as expected. Double clicking a metaframe file from should behave the same way as File>Open in every case.