Now I don't know if I'm asking a question that has already been asked, but I searched around quite a bit without finding a solution that helped me.
So I made a low-poly tree in 3DS Max and this is what happens when I try and use the Terrain Editor to paint my trees on to the terrain:
They get rotated to a 90-degree angle and become super-small, as you can see in comparison to the normal Unity trees.
I have tried everything to fix this, including scaling them up in 3DS Max before exporting, exporting with the object rotated and even increasing the Import Scale in Unity itself.
Here's what it looks like when I import just the tree as an object into the scene:
And here's how the normal trees show up: