I'll give this a shot.
1. In the "map edit" section in Worldforge.....in the drop down list on the bottom right for what tiles you want to place, there is an option to choose "altitude only". Then you type in the change you want in the altitude box and click on the tile on the map that you want to change the altitude for.
2. If you want to add just single trees, here and there, use Axis to place them then static them.
3. Open Axis......On the "settings" tab, click on "Static Tool". Then click on "help" and it'll show you a tutorial of how to use the Static Tool.
And, don't forget to rename the statics0.new and staidx0.new files (which are created by using the Static Tool) to statics0.mul and staidx0.mul and overwrite the ones you were using before. ****ALWAYS make backup copies before overwriting files****
Hope this is helpful.