| For this tutorial I am going to assume that you know how to navigate around the toolset (see here) and how to move around the preview area (see here). I am also going to assume that you know how to create an area (see here) To be able to lay Grass within your area you need to click on the Terrain Tab and then you will need to click on the Grass button as shown below: 
The Brush pane you should be fimilar with as this is virtually the same as the texture brush tools. The only difference is the Density setting, which means how much grass will be laid within your brush size.
The next pane is the Options pane, which you can see below: 
As you can see above you have the Blade Size, which adjusts the size of the grass. There is also an option for the Blade Size Variation, which adjusts the size variation so the grass does not have an even height. The higher the number the more extreme the variation will be. Next we have the Grass pane: 
Here we have the Paint button, which you need to click on and then move to the preview pane and then click on your area to start to paint the grass. Next is the Erase button, which removes any grass that you left click on within the preview pane. Then we have our selection of grasses that we can chose from. To select a grass you can just left click on it and it will be highlighted in Blue (see above), plus there is a scroll bar you can use to select the grass listed lower in the list. You can select up to 3 types of grasses to be laid at once (just left click another 2 types of grass to select them). If you try and select 4 then 1 will be automatically deselected. There does not appear to be a limit on the amount of different grasses that can be painted within a tile.
TIP from Hellcow
While the grass effect in NWN2 is certainly pretty and an easy way to spice up an exterior area there are few things about it that is worth giving some consideration before using it; namely area size. If not used with consideration grass can make your area size explode.
In a test on a blank 8x8 area the result was that the flat, unmodified area had a size of roughly 7 MB, however after the walkable part of the still flat, unmodified area was filled with grass that size had grown to roughly 15 MB, which is more than twice as much; just for grass.
Also there is a limit to how much of each type of grass you can paint in one meta-tile, but there doesn't seem to be a limit to how many different types you can have in one meta-tile; so if you fill the amount for one sort of grass in your meta-tile; you can fill out with another sort.
Lastly a few shots to show the good and bad way to use grass:
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