I couldn't find an answer for this but I discovered it by running TBB Firefox from the command line so I'm putting it here with an answer.
1 Answer
Tor puts user specific configuration in:
~/Library/Application Support/TorBrowser-Data
So for the user guest the torrc file is:
/Users/guest/Library/Application Support/TorBrowser-Data/Tor/torrc
Note that this file will be empty or might not exist unless you have made some changes from the default.