Assuming you are considering it and treating it as a physical part of the rental property, just add the land in the Assets/Depreciation section in Rental & Royalty Income (SCH E). It's classified as rental real estate with the "cost" and "cost of land" being exactly the same so that nothing is depreciated. (Since land isn't depreciable.)
You just work through the Assets/Depreciation section again, elect to edit that particular asset you entered incorrectly the first time so you can work it through and correct it.