and to answer my question with what I know, so far, you can't. Why? Because any desktop or laptop is going to run emulators faster than any Pi. There's LOTS of Pi software that can run on Ubuntu, and lots that can run on OS X (via Homebrew distribution for OSX,, for example). It's a small mental leap then to asking 'How could I run RetroPie on a Ubuntu x86 computer', and then another small jump to 'How could I run under OS X?'. Isn't it reasonable that people on other platforms would want the same convenience? Setting up MOST emulators is painful, but RetroPie makes everything easy. Put it this way: You know that RetroPie source code could be compiled for another CPU, right? Just like any Linux software could.
The poster's question is perfectly understandable to me, and I came here via Google looking for the same answers.
I'm really not sure why everyone is confused.
