First, you need to rescue Ste from Vicoria's cave (Ste is the 2nd prisoner after you rescued Korg from her). And then he will tell you where farmstead is.
Second, check at Forest path (The way to a bridge where the troll is) and a new option 'look for the cave' will appear to be selected. After that you will unlock Plains cave. You just need to enter a cave and explore the underground, then there will be Farmstead Icon on a map (underground).
At this time you can only go to Farmstead by underground way. But after you finished Country road event. The new on ground way would available for you to use it.