1. The center island is left out intentionally, so that you can start a new game or continue your current game with this world map. To do so, make a save game on the center island!
2. In this download zip archive you'll find a complete 'world' folder structure. Unzip this archive and copy the content over your existing 'world' folders, replacing the containing folders (0_0, 0_1, 0_2,...)
There is no folder '2_2', (tile 13, counting from top left ) which would be the center island where you have saved your game. The center island will not be overwritten by this archive!