There are two methods to install these modified maps into your FFXI client.

Copy and Paste the ROM## folders directly into the ./FINAL FANTASY XI/ folder, replacing all files when prompted. This will OVERWRITE all the game dats with the updated map files.

Alternatively, if you use the Ashita launcher, you can install the "Dats" Plugin and use that Plugin to trick the FFXI client to thinking these .dat files are located in the ./FINAL FANTASY XI/ folder without actually replacing any game files. This method is preferred if you frequently update, or want/need to use some form of compatibility .dat files to play on a private server and retail at the same time.