The following guide explains how to generate a world with the Biomes O' Plenty mod if it doesn't by default, in a modpack that contains the mod.

Open the /config/biomesoplenty/terraingen.cfg file via FTP (Files -> FTP File Access). Add the following line above the final '}':

B:"Add Biomes To Default World"=true

After doing so, go to the server.properties file in the root FTP directory and set level-type AND generator-settings to 'BIOMESOP'.

Finally, delete your world folder via FTP while the server is stopped then start it again and your world should be generated with Biomes O' Plenty.