Pixelmon 2.5.1 for minecraft 1.6.4 has been released! You can find the download links (including the required forge)here http://forum.pixelmonmod.com/viewtopic.php?f=6&t=8656.
Due to the overwhelming number of people trying to access the website, we have massively exceeded the amount of bandwidth that this site can handle (Mr M has not even been able to log in). We are working on the problem, but due to being a free mod, there is little we can do about this at the moment.
Latest news and updates are being posted on the official pixelmon twitter, the offical pixlemon thread http://www.minecraftforum.net/topic/1276311-164-pixelmon-251-october-27th-2013, and we are working on another hotfix to address some of the new issues that have appeared.