A heads up if you're trying to run Fiddler on a Mac with mono 5.2. Starting from Mono 5.2, the "mono"command defaults to 64-bit. Use --arch=32 switch to 32-bit mode to run Fiddler.exe.
mono --arch=32 Fiddler.exe
This threw me for a while as I'm not a mono user and Fiddler continually failed on start with the error below.
WARNING: The Carbon driver has not been ported to 64bits, and very few parts of Windows.Forms will work properly, or at all
at <unknown> <0xffffffff>
at (wrapper managed-to-native) System.Windows.Forms.XplatUICarbon.CGDisplayBounds (intptr) [0x00002] in <37dfd86a0d3248c0b8cd4af52ec71426>:0