It's not easy. I can't even get my map to show up offline.
1. - Make sure the terrain name is 8 or less lower case letters.
2. - Put one field on it so you can enter a tower offline.
If you have named it with too many letters, go into the terrain editor and do an export. Then you will generate a heightmap file. Start a new terrain the same size but, with a proper name. Eight lower case characters or less. Do an import and go to the directory of the first terrain where the heightmap file was exported to. It is named hight.raw, import it as a singed file. Then carry on from there offline to look at your terrain. Oh, and you will have to repaint it at this point.
My first oceania project version I named it solomonils and the terrain would never load offline.