You need to create your own mod (option in file tab of the editor - needs a restart before you can tick in the launcher for use). Then in the resource browser window of the editor, select your assets folder. Then, in that empty window right click, which brings a pop up window. Choose Import new asset and select your pgn file (wherever it's located).
The resource browser will copy the pgn file into your asset sources folder and compile and save a *_tex.tpac file in your assets folder. The vista texture will only be selectable in the terrain property browser, if it has been imported by the resource browser to create a tpac file, which the game uses.
As stated in it, my world-mapping document assumes familiarity with the scene editor and Taleworlds Documentation.