This joke has always confused the hell out of me. My very first time I had no trouble finding it. It's very easy to find. It's literally the part that sticks out and is the first thing you get to usually.
Partly the joke is that men are selfish and don't care about the woman's satisfaction during sex. They can't find it cause they aren't looking for it and/or don't want to.
Bruh the girl I'm dating can't find her own clit sometimes because her clit is invisible and doesn't have a bump until she's really REALLY excited (mid sex) kinda funny when she told me haha