What version are you using? Did you add the landscape through Add -> Mesh -> Landscape? Remember that as of blender 2.79, you don't have to install it using a separate file, blender already has it in its optional addons with the compatible version.
Ps. I had some trouble when i was first starting out and downloaded the addon from a previous version, causing compatibility problems.
As described here you could try:
1 -close blender
2 - rename C:\Users\yournamehere\AppData\Roaming\Blender Foundation\Blender\2.79 to C:\Users\yournamehere\AppData\Roaming\Blender Foundation\Blender\2.79.bak
start blender, should be good as new now.
Remember to always download from the official blender site, blender.org