Hi Tongs
To expand on what Spruce has said, compatibility mode is used for running pre TS2009 assets that have problems in 'native' mode. This is things such as trees (that use alpha blending to make them see through), or assets that are 'faulty' in some way. It does however allow assets made for TS2009 to operate in an almost 'native mode' environment at the same time.
As spruce has said, to turn compatibility mode on (you will need it of you want to use trees at this stage), you will need to go to main menu, then options, and you will find compatibility mode in the video options area. You will then need to restart TS2009 for compatibility mode to turn on.
Have fun
Zec