Actually it's easy. the hard part is finding a .obj plant that dosen't crash it on render. On the TG2 download page there is a link to some Xfrog plant that are for TG2.
Click add object,select population, select obj reader or tgo reader; for the Xfrog plants. Now if you have imported a .obj plant click render, if TG2 crashes then that object wont work.
Ok, lets assume TG2 didn't crash
In your default view in the preview window you should see the area bounding box and in the lower left corner is your "field"; the area where you objects will be. Now reset your area center to 4500,0,4500. This will move your field to the center of the area bounding box. Now you could rest your area a and area b to 8000, but note that it takes a lot of instences to fill this area, and your machine may not beable to handle it. Now if you are doing terss and you don't want them too close together set ogject spacing to 50; adjust to suit, but note decreasing spacing increases instances. Now check preview instenses. Don't try to do much with it selected as it really slows you down.
To avoid filling the 8000 square feild. leave the area a and b at 1000. Find your shot, point of view, then click set camera, zoom back out and now you will see where you camera is set up. now rest your area of center till it is in front of the camera. then increase area a and b to fill the shot.
I haven't done it yet, but I supose you could Use density shader, create new shader, surface shaider, surface layer, then goto surface layer and set slope and hight restraints.
I haven't tried it yet beacuse I'm stuborn and I keep trying to render that feid of wheat with the area at 8000sqr, and TG2 keeps saying "I don't think so"
The things that happen while your typing