I have the Hearthfire DLC but there is no dialog option for the land. Any help?

There is no such place in the Heartfire DLC.
Or you just have the name wrong ( there is a mod called Riverside Manor.) But you likely mean http://www.uesp.net/wiki/Skyrim:Lakeview_Manor. Read the second paragraph there to see how/when you can buy the house.

These are the other two houses in Hearthfire.
http://www.uesp.net/wiki/Skyrim:Heljarchen_Hall
http://www.uesp.net/wiki/Skyrim:Windstad_Manor