Ok, I guess I'm 99% sure it's not a hoax, but still, after the frustrations trying to find the darn place, there's still that little doubt in my mind. 
I happened to be in Europe when I read Atcal1's trip report so I said what the heck, this might be my last trip to Europe, might as well try the FKK experience. I really under-estimated the difficulty of finding this place.I got a hotel near Frankfurt central station (btw, this place is a bigger scene than Amsterdam). I rented a car and left for Altenstadt around 10:30. Got a highway map of the "Greater" Frankfurt area with all the surrounding cities from the rental company. Altenstadt is not on the map. No problem, just follow Atcal1's instruction. Getting out of the city and onto the autobahn was enough trouble for me since because German names are hard to memorize and there no direction(north,south..etc) to the highway signs. So 30 minutes later, I'm on A66 (should only take 5-10) minutes. Few minutes later, A66 turns into a local express way. I thought a made a exit by accident so I went back and circled a few times, then I stopped to look at the map, and sure enough A66 turns in to 8 on the map. 8 is about 20-30km before it turns back into A66 again, weird, I thought I was in bizzarro world. I better follow my map. Atcal1 traveled from FRankfurt airport so I thought that was the difference. Then I got lost again because there was a local 45 on the map and I mistook that for the A45 in Atcal's instruction. I spend 45 minutes there. Finally got to A45, and the drive on A45 is a long one. Probably around 30min, well, it seems long at the time because I was already very tired.
When I got to Altenstadt, that's when the fun began, to make a long story short, I spend 2 hours there and never stepped into Atlantis door. I followed Atcal1 instruction to the tee, but never found a sign that says Siemenstratte. And that final right turn, takes a trememdous leap of faith like Indiana Jones looking for the Hole Grail. I took that leap but found place that looks like a factory with a industrial iron gate and a small lifting crane. There was some warning sign on the gate in German, no door bell or phone. I was too tired at that point to knock on the iron gate in a foreign country. There are actually 3 different right turns you can take in that area, couple of then lead to nowhere, but I got to drive around a empty dirt field in pitch darkness for a while. Now my car is coverd in mud and I probably have to pay extra for cleaning. In the end, I wondered, if this is not a hoax, why would a gentlemen's club be so hard to find.
Getting back to my hotel was another adventure, but I wouldn't bore you guys with that. I got back around to my hotel 3:30am
It was an interesting experience, I should have done more research on my own to get prepared. To everyone who's thinking about Atlantis, read as many trip reports as possible, and do your own research like Atcal1 did. Can't believe the length I had to go through for some pu$@y. The weird thing is, I'll probably try something like in the future. (In fact, I'm thinking about taking the train to Oase tonite per Atcal1's instruction
I guess I'll never learn.