Most of the maps in RTW weren't included in BI. Helping players to surmount this obstacle, the custom_locations file has been edited to include all of the RTW maps, and a bunch of additional ones to boot - including a new grassy flatland map!
To install the map pack, simply put the file in the RTW and BI data folders.
It is strongly encouraged to make a back-up of the original custom_locations file before applying the updated version.