How to use: (thanks to ALF at tycoonplanet.com for the trees, sorry I didn't ask before using them)

Just open a scenario in DImport, look at the slots until you find an empty one, then click Scenario\Load DAT and open the DAT file. Save it with a different filename (also change the scenario name and/or description so you can tell it apart).
You will have to restart RCT2 to get the new scenario to show up.

It's also possible to import other types of objects, just click on the Scenario menu and choose the object type.

Then, open your scenario in the game and it should have the new scenery object available