I just thought I should provide a little update with the status of this. The Librarian software is written in Java, which is generally great as a cross-platform environment. However the Atmegatron uses SysEx to transfer messages to and from the Librarian and Java on OSX doesn't support SysEx (no idea why!). Luckily this developer kindly provided a workaround for this and up until now the Librarian worked just fine under OSX.
However with the latest OSX/Java versions that workaround actually prevents the software from opening. This is because it hangs when it tries to access the MIDI devices. Full details of the bug can be found here.
So sadly for the moment my hands are tied. Ultimately I think the solution is to port the Librarian to another cross-platform environment, possible something like JUCE and I'll post updates with the progress.
Please feel free to post questions about this issue on the forum and I'll do my best to help.