You need custom script, you can find by using search option in forum. :}
http://emudevs.com/search.php?searchid=296029
Crossfaction battlegrounds is a script, if you are using a repack check and make sure the repack has the script. You could just check your worldserver configuration file also (would be near the bottom). If you are not using a repack then you might need to find and update a crossfaction battleground script and add it to your source.
is there a way to enable the cross faction bg?