Note: you must be signed into your Google account. To add a farm first click (while viewing vendors) new page which is in the top right corner then name the farm your adding and make sure to hit put under vendors. Then add a description and I recommend adding only two sentences then add a link to your farm's website.
Then click on more in the top right corner. In more click on manage site. After that you will see a bar on the left, scroll down to site layout. In site layout you will see edit horizontal nav content, click on that.
This should come up:
To make this page more accessible Click add page and then type the name of the page that you created for your farm. After that hit OK. Then use the arrows to put you farm under Vendors in the right alphabetical order. After click OK and then save on the site management.