I was wondering if i can use teragen terains in unity.
Not directly (i.e. Unity does not read .ter files).
You could try with TG2s LWO export, but that will give you a mesh, not a terrain object.
Nevertheless, TG2 is my tool of choice, and if you’ve seen my recent posts (e.g. http://forum.unity3d.com/viewtopic.php?t=30317) - these are all from TG2 terrains.
My method for creating heightmaps is described on the Planetside/Terragen forum here:
You then need to convert it into .raw and you’re done.