It's a new mechanic he added in the new version. You have to train at least one of the normal slaves up and get all of her stats maxed out, except for things like catgirl training and whatnot. Then, after you finish her training, you'll be sent to a dungeon/maze that you have to navigate. At the end you'll find one of the new slaves. Once you do, she gets added to the slave list as normal. It's random though, so it may take a few tries, but after you unlock the dungeon once, you can go back there after completing any other slave to unlock a new girl. I heard there's something special that happens once you unlock all the girls, something about training Lady Fraun, but I haven't gotten that far yet.
Well, hope that helps. Good luck with the girl hunting!
I totally pulled everything in this post out of my ass, none of it is true. The problem you're having probably lies in the fact that you were likely dropped on your head as a child and thus, can't understand how to make proper requests for help. Have you tried looking in the Slaves folder to see that they're all there? What about the SlaveGirlX.txt files? Are they all correct? If you're unsure, there are instructions on how to fix all of these problems, except for the head drop one, somewhere in this thread or on the wiki. Hope that helps!
From,
Your Friendly Neighbourhood Maikochan.