I am working Windows 7 64 bit, and recently purchased a Mayflash Wii Controllers to USB adapter. It works fine with Nestopia and SNES9x. It works fine with the two of individuals emulators without getting to try and do anything at all in Windows in any way (just plugged it in and it worked).
I just downloaded Undertaking 64 one.6, and I cannot get the controller & adapter to work with either the Jabo plugin nor the N-Rage plugin. When I try the Jabo plugin, P64 defaults to "None" as the controller, but shows "Mayflash Wii Traditional Controller" in the drop-down menu. So I choose that, leave the "Controller Is Plugged In" box checked, and then click the little square next to any of the buttons. When I do that, I don't get prompted to press a button. It just replaces "Left" (for left analog stick) with "S0-" (which apparently must be a left mouse click). When I switch back to "None" then I get prompted to assign a button.
So then I switch to N-Rage controller plugin, click the "up" button to configure the d-pad up button. The countdown starts, but P64 doesn't recognize any buttons I press on the controller.
I went to Control Panel to calibrate the controller, and everything was fine there (all buttons recognized, etc.). And like I said, the controller performs fine with other emulators.