Yo ! Several options:
1. Use the export mesh node to export your terrain as a mesh. Then, simply import the mesh to Unity and put it in the hierarchy !
2. Export the heightmap as a png ; then, in Unity use this script:
http://wiki.unity3d.com/index.php?title=HeightmapFromTexture (the way it works is in the link). Nonetheless, there might be a loss of detail because of the conversion ; apart from this potential loss of detail, the terrain might not look as in InstantTerra. And that is normal. Check if the size of the iTerra terrain is the one from Unity's terrain. If not: scale the Unity's terrain to the size it has in InstantTerra from the setting tab of the Unity's terrain. You can also create an iTerra terrain which is the size of the default Unity's terrain, and there will be no problem ! (at least shouldn't !)
3, which is not really a 3. Check the answer above about the raw format ;)
Off the top of my mind, these are the two ways I have. If anyone has other option(s), feel free to share it/them too ! :)
superusername