Am not sure the expand plot ever becomes enabled.. having to disable the auto replant to switch the crop you may also want to clarify. It's pretty and fun though!
To enable the expand plot, the first tier must be researched in "Plot Management". I will work on making that more obvious though, as I can understand the confusion for sure. I can also have it clarify turning off auto replant to switch the crops too. Thank you so much for bringing my attention to this and for the kind words!
EDIT: The entire system for this has been overhauled. You no longer need to research the "Plot Management" tier to begin expanding plots. Now all you need to do is have the money to purchase them!