That is strange, wondering where they are for Mac. The default situation is that Interactions folder is copied from the Gallery to Public documents (called Shared in Mac). Those are used in projects, and also updated regularly because when an interaction gets a patch or a new version it is pushed to the CP-user. In my Interactions folder (here a screenshot) you see that there are some backup files, which were older versions of that interaction:

It's possible I found them here: Adobe Captivate 9-->Gallery--->Interactions. (see screenshot).

At any rate, I did find the solution to my problem in a different forum post from Rick "Captiv8r":
