I have never had a problem getting Terragen files into Wilbur and from there to WrpEdit.
I made a tutorial to explain the process a while ago now.
It is available in the Editors Depot....Tutorials.......Third Party Tools.
It is called DEM-WrpEdit Tutorial.
It includes the steps from Terragen file to Lon/Lat Text file and from there into WrpEdit.
Planck