The add-on is installed as a regular add-on in Blender by installing the zip file as it is and not extracting in beforehand. The interface can then be found under properties in the 3D view. 

To use the add-on, you need some sort of mesh object where the grass should "grow". This can be for example an A.N.T. Landscape or simillar. After you have chosen your "target object", you can choose which type of grass you want and then simply pressing "Add grass". Thereafter, you have the choice of adding masks by pressing "Use masks". The three masks available are:

Camera: This mask lets you make the grass to only grow within your active camera visible range in order to reduce CPU and memory. Note that this is the only mask that don't auto-update meaning you have to press the "Update" besides the camera tick box.

Slope: Since grass very seldom grow on cliffs and steep hills, this mask allows you to prevent exactly that. You also get to set the two thresholds. The thresholds goes from zero to one where zero is maximum steepness and one is flat. The grass will grow at a maximum over the high threshold and not at all below the low threshold. If you for some reason want the opposite, you can let the high threshold be lower than the low threshold instead. The threshold values chage the grass distribution instantaneously.

Altitude: Altitude is a mask which lets the grass only grow within a certain altitude range based on the altitude of the mesh. This is very useful when you want for example no grass below a water surface or not on top of a mountain or both at the same time. This is, simillar to slope, decided by the high and low thresholds.

The masks can be used one or two or all three at the same time independently.

After the grass have been added, you can also set the number of grass particles and the scale directly in the add-on.

If you want to change grass type after it has been added, you can still switch by choosing another grass type and press "Update". You can also add another grass type on top of another. However, some features may not be changeable on the first layer. You can also delete the grass by just pressing "Remove".

The grassgenerator can be chosen as a type and that gives you som other options in order to generate new grass objects. The generator lets you decide how many straws you want in a clump of grass giving density to the grass. You can also choose how many clumps you want, more giving increased variation to the grass. The number of loopcuts decides how many polygons one straw is made of, a higher number gets increasing detail. Length/width ratio decides how long the straws should be compared to its width. Maximum bend range decides how much each straw can possible bend compared to its length. If the box "Use material" is checked, a procedual node tree will be assigned which can be further more edited in the node tree. Note that by updating the grassgenerator resets the material so that should be made last, or a duplicate can also be made to prevent that.