When in Editor with your level open (top down view)...
Hit test run to run your game so you are in your 3D game.....
Then Hit Tab key twice to bring up your game settings and slider controls. You will see controls and settings in there for many things - including controlling your grass height, width, density and so on.
Make your changes and use Esc key to close settings and controls.
Note : When changing Grass (veg) settings via sliders please be patient as GG will take a short while while it recomputes things.
Hope that helps.
System : Advent Windows 7 Home Premium 64bit. Intel(R). i5-2310 CPU @ 2.90GHz. 8.00 GB RAM DDR3. Storage : 1389 GB : 1088 GB Free (1389 GB Total). Graphics : NVIDIA GeForce GT 530. Total Available Graphics Memory : 4095 MB. Dedicated : 1024 MB. Shared : 3071 MB. Direct X10