Thanks to Llam4  for unlocking this little secret.

To install the Secret Island, unzip all of the files to the same folder and then click on the installer.bat file then sit back while it finds the Black & White 2 folder and makes the changes.

To use, you must start a new game. So make a new profile just for this map.

In order to start a new regular game, you need to click on the restore.bat file and the original files will be restored.

To install manually: 
Land10.bwe goes into the C:\Program Files\Lionhead Studios\Black & White 2\Data\landscapes\BW2 folder 
map.txt goes into the C:\Program Files\Lionhead Studios\Black & White 2\Scripts folder.
 
**Do rename the orignal files or back then up before adding the new ones.**

Note: the install script will rename the original files using "backup_SecretIsle_" as a prefix if you need to manually uninstall 