I've downloaded the hi res terrains for Earth, Moon and Mars but I have no idea what to do with the resulting ".tree" files...
Any ideas?
You need to put them into their respective folders in the Textures directory of the main install. You can either add a .old to the low res versions, or completely overwrite them. Here's a link to the installation instructions for the hi-res files:
http://web1.orbiter.us/tex_help.html#tex_installOrbiter always intrigued me, but when it came down to playing it, it was just too dry for me. I ended up learning a lot more about Space exploration from KSP. I'm sure the physics etc are far more authentic in Orbiter though.
Different strokes for different folks. I actually prefer the no-nonsense and engineering feel of Orbiter, and found KSP to be a little to kitschy for me. I also like that Orbiter is "real." When you fly through the solar system, you're flying through ours, not a fictional one. Especially with the hi-res terrains with the new release. The planets have never been more beautiful, especially Earth.