On OSX you execute it like so:
"mono TNServer.exe"
^^ 100% accurate... perhaps 10% helpful

Instructions to run TNServer.exe on an OS X (Mac) computer:
1: Download the Mono RUNTIME Engine from
THIS PAGENote:
Direct link (may not work in the future)
2: Install the MRE (the .pkg file you just downloaded)
3: Open a Terminal window (can be found in the Applications/Utilities folder)
4: Confirm "mono" is a valid command by typing "mono" (without quotes) and press enter. If installed, lots of information will be displayed; if not "command not found" error will display <- you will have to fix this in order to continue
5: Type "mono " (without quotes AND INCLUDE A SPACE). Now find your TNServer.exe file and drag and drop into terminal window. The entire path should be displayed to your TNServer.exe file
6: Click back in the Terminal window and press ENTER
If successful you will see the server info along with a "press q to quit" which, if you type "q" (without quotes) into the terminal window and press enter, the server will save and shutdown.