They're called 'free' but you still need to pay for the upgrades. Make sure you have the gold to get it. Don't worry, I was slightly confused about it at first too. xP
Oh, thanks - so are you saying that if I didn't have those upgrades, it wouldn't matter if I had the cash for the upgrades I still couldn't do them? In other words, I need the upgrade points AND the cash?
Next question - I have a slave who still has attribute points to assign, but is out of slots for them (extra attribute points). Will she have the chance in the future to use them or is she now at her max potential?
That is correct. When you click on an upgrade, it should, to the left, give a description. It'll tell you the required points in green, and then mention the gold needed in yellow. You need both the money AND the points. It gets expensive :/
At the moment, there are very few ways to increase the maximum attributes. If you build a Laboratory you can use it to up a few attributes. There's a surgery/experiment for increasing Agility, one for Strength, Beauty, Awareness, Oral Sex skill, and, if they have scales/fur, one for Defense. You can change a few other things there as well, but they're pretty much all cosmetic and don't really affect anything.
Other than that, I know of no way to increase base attributes. So, if you haven't already, get a lab, assign a girl as lab assistant, and upgrade the girl you want. It requires Mana, time, and usually items. Don't spam it though, give a slave some time between operations. There's a feature in the game where if you use too many potions or perform too many operations in the lab on a single slave within a certain amount of time, they might end up mutating. Sebastion, who you meet during a quest where you need to make a potion, explains it.
So, yes, you will have extras, but at the moment, you'll only be able to increase Strength and Agility via the lab, both of which go up by 2, meaning anything above 4 extra points are a waste of time. When you have 4+ points you can't use on an unmodified slave, just ignore their levels from then on, unless you feel you just want to.