I also had this "bug", and pressing F wasn't doing anything. I tried it with no other mods installed besides xSkills, and then it was working. Which means there was another mod I was using that was screwing it up. After a bunch of trial and error I figured out that it was Primitive Survival. Primitive Survival does a json patch on one of the hoe's behaviors, which causes the Extensive Farming to not work anymore. The behavior that Primitive Survival adds is the ability to create furrowed land with the hoe. You have to disable that if you want to be able to use Extensive Farming (which is way more useful imo). So go into your modconfig for Primitive Survival and set "FurrowedLandEnabled" to false.
tl;dr
If you're having this issue, and you're using Primitive Survival with xSkills, then you need to set "FurrowedLandEnabled" to false in your mod config. Otherwise the Extensive Farming skill won't work.