Originally Mac keyboard layouts came in the form of .rsrc files. Unfortunately these sometimes no longer work in the latest versions of OS X, or don't work for all apps. Thanks to info on the Ukelele site, I've located a way to convert these to the newer .keylayout format.
Download and install the Apple Font Tool package from here. Then put your old .rsrc keyboard file in your Home directory, open Terminal, and type
If all goes well, you should find a new file, name.rsrc.keylayout, in your Home directory. There will also be an .icns file if your old layout had an icon in it. If that command doesn't work, you can also try