How would I remove or add grass to the land?

You use the landscape editor and either use the 'grass' texture or the 'nograss' texture to paint the land.

Oh, alright. How do I paint the land? When I try, it just covers the ground with a black shade. It's right click, right?

Does no one know? Seems really simple....

If it were simple, you wouldn't have had to ask.

Black means you have more than 6 texture in a quad. (of a cell) You can press 'B' to see the cell borders.
Quad means there are 4 sections in a cell.

When in 'H'eightmap editing, make the render window active and press 'I'. This will bring up all the textures your using in the current cell your camera is in.
You can remove textures, or replace textures.

It also could be you are 'shading' instead of 'coloring'. You should check out the landscape tutorials.