you don't you purchase french or hungarian parsley seed oil from an essential oil company that's been GC-MS tested so you know it contains 30-50% myristicin before you purchase and then distill under vacuum to separate myristicin, elemicin, apiole and the various terpenoids (keep the first 3). You would need several kgs to steam distill a few mls of oil from nutmeg and it would likely be 10% myristicin at most. AS far as I know coconuts do not contain myristicin...
Why MMDMA and not MMDA? I've read mmdma is not very active or as interesting as mmda but haven't tried either myself.