This is from RoyS, posted to the Unity forums in this thread, with tiny changes to update the workflow to the new tool version:
BTW, here is my Tutorial for using Tom's Terrain Tools (the first version) with L3DT (I use Pro)…
Here's the steps I use to get my terrain into Unity.
1 — Generate your L3DT map via the Wizard if you're unfamiliar with it. Then…
File-->Export map-->(select) Heightfield File format - (pulldown) select "RAW" -type in a File Name -check "resize for export" -1025 size (default) Operations-->Alpha Map-->Alpha Express (png)
The raw is your heightmap and the png is your splatmap. Move those to your Assets folder.
2 — Make sure "Tom's Terrain Tools" is installed per his instructions.
3 — In Unity
Terrain-->Import Heightmap RAW --Bit16 --Windows
Now you should see your heightmap in your first colour chosen. If the mountains are too high, then go Terrain—>Set Resolution and change the 600 value. Just change those numbers until you get the desired height of your mountains and valleys.
Now for the rest of the colour of your terrain.
In Unity, select your png texture in the Project folder and in Inspector change it to RGB 24 bit, set the "Read/Write enabled" box and hit the Apply button. Now you won't get an error message to change it to 24 bit RBG as you've just done it. Now to add the splatmap…
Window-->Terrain Tool and drag the splatmap into the first splatmap field.
If you're sand is in your mountain area and your grass under the sea, then just change the order of your textures until it's right (ie, the terrain paintbrush textures as if you were painting one from scratch — the same ones you loaded above).