The adapters I bought last year were the same ones Radio Shack were selling, and my Mac didn't see 'em at all - I guess I got a bad batch. My PC was able to see 'em, but the company that sold them to me (Lik-Sang.com) said "we advertised them as being PC-compatible, so we only support them with PCs" and refused to take them back. (Mine were translucent green, model XK-PC2003 - maybe Radio Shack's were a slightly different model?)
I found an app named Joystick And Gamepad Tester
which lets me test exactly what values my Playstation 2 controller is giving through the new adapter I'm trying. (The pair I bought from Radio Shack today are these
With analog mode turned on, it sees the D-pad as a hatswitch returning 0 thru 7 for the directions north thru northwest (clockwise), and 15 at rest. With analog mode turned off, the D-pad behaves as an analog stick, returning either 0 or 255 in either direction, or 127 at rest. It even sees my Dance Dance Revolution dance pad as an analog stick in the same way! Because it treats left/right as a single control, it's not capable of recognizing the left and right arrows stepped on at the same time. (Same with up and down.) So it looks like this adapter is being stupid - unless there's some app which can remap the controls to something more sensible which MacMAME can handle, I'll take 'em back to the store.
(It occurs to me: how STUPID does an adapter have to be to try to map a D-pad as an analog control, or a hatswitch, instead of as a D-padl?! The adapter knows full well what input was triggered on the controller, and it has its choice of how to report this to the host computer... why would ANY designer in his right mind report it as anything but a D-pad?!)
I really wish Apple would hurry up and port InputSprockets to Mac OS X.