Well, it IS straight off the proverbial oven...
Killing two birds with one stone and marking this under "Nonfunctioning." PicoDrive, you have a friend now.
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Eyyy, we're the ONLY thread now! : D
...Go on.
And you'd be right in being confused! By default, RetroArch's key combo is, for some reason, L1 + R1 + Select + Start.
If you navigate to RetroArch -> Settings -> Input, you can change the Quick Menu pullup combo there. I personally use L1 + R1 + Down + Y (Square).
...Sounds like, however, you're new to Libretro as a whole. You'll need some time to adjust to the menus, but it's worth it as everything performs (so far) infinitely better than any standalone emulator available for HENkaku.