Might sound daft, but did you click on the arrow at the side, adn then scroll it further down for Dx11.
The tesselation option does show up when Dx11 is selected (appears amongst the volumetric lighting etc... options), but isn't there when in anything less.
Btw, does anyone think the Tesselation is working, as that first guy you come across in the mask (who gives you the co-ordinates for the ship), doesn't look tesselated to me, those things sticking out the sides of his mask still don't look round to me with it on.
Tesselation isn't automatic, its not like AA and it just applies itself to everything.
THe most simple way to describe it would be, if a designer creates some texture, or a character, they'd normally create it in high detail, then scale it down massively, cutting the size and quality, so it could work in the engine with the hardware available. Tesselation is basically a way to save the high quality version, at a very small size, and use hardware to use the small amount of info saved to basically recreate the high quality image.
The high quality images/textures HAVE to be designed, it doesn't make anything crap and badly designed look uber pretty.
Either way, supposedly at top settings I was thoroughly dissappointed, I thought the first Stalker looked very dated, I think this one feels even more dated. The biggest reason in the first was overuse of HDR as it was new, and sacrificing the quality of everything else, and the horrendously poor grass/ground done in very poor quality sprites gives far to much of the screen real estate a decade old game feel.
The first 10 minutes had a crap fight where I learned the guns still do cone fire for no reason, dodgey animation on the first two animals I saw, who were dumb as crap. Some insanely stupid bandits, horrible and generic conversations that have no fluidity to the conversation.
Completely and utterly unimpressed so far, but will persevere, maybe not for too long though. The first game had a few fantastic bits, but was overall a massive let down, this feels like its going the same way at the start, hopefully with a few great fights later on, but it feels so old and badly put together. Didn't both Stalkers sell quite a bit, at some stage some investment in a bigger team to improve the severely lacking parts of the game should happen. Some of the poor parts of the first game are forgiveable for a first real release from a studio with little cash, the 3rd game in the series doesn't get the same leeway in my opinion.