Did you speak to the alchemist and he told you about the cave? The cut scene should be automatic as you approach the rock (#4 on
this map).
Did you find the other ingredients and move them in your inventory before speaking to the alchemist? That could cause a problem in an earlier version, but I thought the
patch fixed that.
If you did run into this problem, there is a workaround;
[*] Complete all other quests and save the game.
[*] If you have a save before finding the alchemist's ingredients and moving them, copy the 2 story.* files from that save into your latest save, otherwise you can use
these files from a save at the beginning of the imp village.
[*] Talk to Altoflix in the north of the village (if required), then the chieftain's son to get the quest to find the alchemist, talk to the alchemist once (only) and head to the cave.
[*] Continue with the game.