Moose,
They've gone to your Master Keyword list. Your Master Keyword list is found on the right hand side of the main Keywords dialog that is accessed from the popup menu on the right side of the Keywords field in the IPTC dialogs. Click on the popup menu and choose Edit Keywords... from the popup menu. Then you can use the Export button.
Or if you want to export and import a number of settings all at once, use the Export and Import buttons in the Preferences dialog. This feature allows you to export and import nearly all settings in Photo Mechanic including Keywords.
HTH,
-Kirk