Ok i have both the parry and riposte skills but i don't know how to use them, i hold block and thats all he does is block, and to be honest block doesn't even work half the time, half the time i hit block he doesn't raise his sword to block and i seem to have to wait or spam the key to get him to do it???
You have to hold down the block key ('E' on my keyboard), though sometimes I've also be able to time it right and hit the block key at the same time as an enemy was swinging at me and it blocked the strike. When you block a strike you also have to then re-hold the block key to get into blocking stance again.
For parrying you hold block and wait until a sword icon comes up on the targeted enemy, if you click right when you see that (I always think I'm hitting too late because the icon doesn't stay on the enemy very long) Geralt will perform a parry move at that enemy. It works most of the time for me, though I don't know why the parry icon comes up sometimes and not other times.
Hope that helps.