Are you importing this into an existing project? Make sure you're not already using variables that need to be reserved for the farming system, and make sure you're setting the region for farm tiles to 97.
You can change the appearance of the UI by adjusting Common Event 117: Tools UI. You can hide it by using erase picture after waiting a period of for the tool buttons to stop being pressed, or something similar for how you want it to appear.
I'm not at my computer right now but you should be able to find a list on the RPG Maker Web forums for script calls for button inputs, or there are plugins that let you use the full keyboard I believe.