Hi
I have installed CivWillard in my Steam Civ5. I see a similar pop-up to the one in your screen shot, but I can click the Close button at the bottom right to dismiss it, or I can toggle it by clicking any of the three buttons at the bottom of the screen.
I have enabled logging in my config.ini file, and the lua.log file contains:
[3578172.279] Runtime Error: xxxxx\Documents\Aspyr\Sid Meier's Civilization 5\MODS\CivWillard (v 1)\CityData.lua:64: attempt to call method 'GetRouteGoldTimes100' (a nil value)
Not being familiar with the Lua scripting language, I can't identify the cause of this error. or try to debug it.
FireTuner is PC-only, as it comes with the Windows-only SDK. I believe it may be possible to use the Firefox debugger to access the same information, but I don't know the details.
I'd be surprised if the problem is specific to the Mac version.