The selection is for what you plan to create, not what you're planting with. Select Carrot and you'll notice at the bottom of the screen that it requires one Carrot seed to perform. That's you planting the seed to get a carrot. Once you have a carrot, you can either eat it, cook something with it, or select Seed-Carrot in the cultivator menu (and notice how it has a requirement of one Carrot to perform) to plant it back in the ground and eventually become three Carrot seeds, which you can then plant to get more Carrots.