Unless I'm crazy, this seems like a simple little braindead bug. You create a new project, a Xam Mac Library project. And by default, the compile target is set to "Executable" instead of "Library." So the first time you compile, you get:
Program 'programname.exe' does not contain a static 'Main' method suitable for an entry point
So you say to yourself, "That's dumb." And you right-click the libary, options, general, compile target, library. And move on.
The first time you encounter this problem, you think to yourself, "Did they do that for a reason? Am I causing all kinds of problems now by changing it?" And after the 3rd time, you think to yourself, "Oh yeah, I forgot about that stupid bug." Somebody should report it to them.