To be honest I also struggled a little with the movement ":)..
For you to interact with the cauldron you have to click Space, while holding an ingredient, and click E to mix the ingredients together.
I guess we didn't really explain it, but we'll definitely work on that when the jam is done.
I hope this helps, and that you can enjoy the game more.
(other helpful tips: when the mix is done, get a potion flask and click Space to get the mix, and then click Space on the delivery table to deliver the potion. You have to cut the shrooms by holding the ingredient with E, and then clicking E on the cutting table. You have to heat the firefly by holding the ingredient with E, and then clicking E on the pan.)