INSTALLATION:

Edit/Preferences/add ons   click on "install from file" and choose the python file included in the purchase "erosion_2_91+.py"

If you need it for older versions, you can install the python file

"erosion_nm-8.py"  

The menu is in the n panel (shortcut "n") of the 3D viewport, in the "NM-8" tab.

USAGE - step by step guide:

NEW:

• "Size" and the "new landscape" button are for generating a random landscape from scratch. But you can apply Erosion onto any mesh.
  The size also acts as a random seed parameter for the distribution of hills and valleys. So a size of 118% will generate a completely different landscape than a size of 119%
  
  

PROFILES:

height, erode, gather curves to adjust the shape of the landscape. if you click the checkbox, the according curve appears

The left part is the lower valleys, and the right part represents the peaks.
A usual rocky/bulgy landscape would have a diagonal straight line.
Meaning at a steepness of 45° rocks start to shift downhill.

normal height curve (left), canyons (middle) and peaky mountains (right)

The height-curve can be thought of as a profile cut out of the landscape.

In erosion LITE there is only a height profile curve.
In the full erosion menu you can also choose a curve for erosion and gather to control how much the mesh is eroded in the lowest and highest areas, as well as how much the mesh gets lifted up, if erosion gathers in an area.

For example this helps to quickly achieve a smoother look in valleys, while peaks stay ridged and rocky.

ERODE & VPAINT:

• "selected" erode only selected vertices.
  
• "border" will keep all mesh-border-vertices in place, for tiling purposes.
• "forest" will generate particles for trees or rocks.
• "reshape" check this if you want to change the mesh, uncheck if you just need   vertex colors.
• "optimize" will subdivide and decimate the mesh to get an efficient topology. 
• "vertcount" limits the optimization to avoid too high polycount.
• "3D erosion" change the mesh and create vertex groups and vertex colors
• "tex res" the texture resolution for the flow texture to be rendered
  
• "2D erosion" create the flowmap nodegroup with the flow texture
  
  

RIVERS & LAKE MESHES

• "river heads" sets how many random vertices in higher areas will be the source for    rivers. "selection" checked will only use selected vertices as river heads.
• "iteration" how many vertices will be covered by water.
• "rivers & lakes" create the mesh from the parts that are filled with water.