Hi Bob,
Thanks for reporting. Unfortunately I'm not able to test it on Mac. Although I have Mac users for Multi-User version of add-on, but they didn't report any bugs. Anyway try the new version:
messagenotesplus_1.4.0.1.xpiAlso you may try to update Thunderbird to 15.0 version. I do not test 12.0 anymore.
Also you may try to enable debugging option in addon's option. Then open Error console (Ctrl+Shift+J). Then if you scroll emails' list you should see in Error console appearing lines containing "quering note statuses pack".
If you do not see this lines, then the most probable reason of the bug is in your system or Thunderbird. If you see these lines, then the most probable reason of the bug is in my code.
By the way, where have you found my add-on? Version with gray default icons is not available yet on addons.mozilla.org.